A shaligram, also called a shalagram shila (Devanagari: शालग्राम शिला Śālagrāma-śilā), is a particular variety of black stone collected from riverbed or banks of the Kali Gandaki, a tributary of the Gandaki River in Nepal, used as a non-anthropomorphic representation of Vishnu by some Hindus. They are typically fossils of ammonite shells from the Devonian-Cretaceous period of 400 to 66 million years ago. The black stones are considered holy because they bear symbols associated with Vishnu naturally, without any human intervention.
According to the Devi Bhagavata Purana, Brahmavaivarta Purana & Shiva Purana, Shalagrama Shilas originated due to the following chain of events.
A king named Vrishadhvaja had been cursed by Surya to endure poverty due to his reluctance to worship any deity other than Lord Shiva. To regain back the lost prosperity, his grandsons Dharmadhvaja & Kusadhvaja performed austerities to propitiate Goddess Lakshmi, the presiding deity of prosperity. Pleased with the austerities, She granted them prosperity & boon of being born as their daughters. Accordingly Lakshmi incarnated as Vedavati, the daughter of Kusadhvaja & Tulasi, the daughter of Dharmadhvaja. Tulasi went to Badarikashrama to perform austerities in order to gain Lord Vishnu as her husband, but was informed by Lord Brahma that she wouldn't get Vishnu as her husband in that life, rather she would have to marry the Danava named Shankhachuda.
In his previous birth, Shankhachuda was Sudama, an attendant of Krishna, a manifestation of Vishnu residing in Goloka, created out of his body who was cursed by Radha to be born as a Danava. As a result, Shankhachuda was virtuous & pious by nature and he was devoted to Vishnu. He married Tulasi on the command of Lord Brahma as per the rules of the Gandharva marriage. After Shankhachuda's marriage, the Danavas under his leadership waged a battle against their natural enemies, the Devas, in which they won due to merit of Shankhachuda's virtue. The Devas were subsequently driven out of Svarga by the victorious Danavas.
Demoralised & defeated, the Devas approached Lord Vishnu, who told them that Shankhachuda was destined to be killed by Lord Shiva. On being requested by the Devas, Lord Shiva along with His attendants & the Devas, waged a battle against the Danavas led by Shankhachuda. However, neither side was able to outpower the other. An unembodied voice told Lord Shiva that by the boon of Lord Brahma, Shankhachuda was invincible in combat as long as he wore his armour & his wife's chastity wasn't violated.
So Lord Vishnu, assuming the form of an old brahmin, asked from Shankhachuda his armour in alms. Shankhachuda donated his armour to him. When he was busy fighting with Lord Shiva, Lord Vishnu, wearing Shankhachuda's armour, assumed the form of Shankhachuda & cohabited with Tulasi. Thus Tulasi's chastity had been broken & Shankhachuda was killed by Lord Shiva's trident, thereby reliving Sudama from the curse.
At the moment of Shankhachuda's death, Tulasi became suspicious that the man who was with her at that time wasn't Shankhachuda. When she came to know that it was Lord Vishnu who had violated her, she cursed Him to become a stone as He had been emotionless like a stone in accomplishing the death of his devotee Shankhachuda & violating her, who was also devoted to Him. Lord Vishnu consoled Tulasi by stating that it was the result of her austerities performed in the past in order to gain Him as her husband, & that she would again become His wife on casting off her body. Thus Lakshmi cast off the body of Tulasi & assumed a new form (which became known by the name of Tulasi). The discarded body of Tulasi was transformed into the Gandaki river, & from her hairs emerged the tulasi shrub. Lord Vishnu, on being cursed by Tulasi, assumed the form of a large rocky mountain known as Shalagrama, on the banks of the Gandaki river where vajrakita, a type of worms having teeth as strong as vajra carve out various markings on His body. The stones carved by vajrakita which fall down from the surface of that mountain into the Gandaki river, become known as Shalagrama shilas.
Historically, the use of Shalagrama shilas in worship can be traced to the time of Adi Shankara through the latter's works. Specifically, his commentary to the verse 1.6.1 in Taittiriya Upanishad and his commentary to the verse 1.3.14 of the Brahma Sutras suggest that the use of Shalagrama shila in the worship of Vishnu has been a well-known Hindu practice. A good number of false shalagrama shilas too remain in circulation.
The statue of Lord Vishnu in the Padmanabhaswamy Temple of Thiruvananthapuram & Badrinath Temple of Garhwal region, & that of Lord Krishna in Krishna Matha of Udupi & Radha Raman Temple of Vrindavana are also believed to be made from Shalagrama shilas.
Shalagrama shilas are obtained from the area of Nepal known as Shalagrama kshetra which lies on the basin of the Kaligandaki river between Damodara kunda situated south of the mountain peak named Damodara Himal near Dhaulagiri (which can approached after trekking for a few days from Muktinath) in Mustang district in the north to Ruru Kshetra (referred to as the hermitage of Sage Pulaha in Varaha Purana) in the south.
Quoting from sources like Viramitrodaya, Chaturvarga-chintamani, Matsyasukta, Vaishvanara Samhita, Puranasamgraha etc. the Pranatoshani Tantra cites the following varieties of Shalagrama Shilas  on the basis of shape, colour, features of the chakra(imprint of the ammonite shell present on the outer surface or inside the stone, resembling Sudarshana Chakra, the main weapon of Vishnu) & mukha/vadana/dvara(a large aperture resembling an open mouth through which the chakra present within the stone can be seen from outside) & distinct markings like the vanamala(a linear white line running throughout the body of the stone, resembling a garland)
● Keshava: i) Marked with a single chakra, a vanamala and 4 bindus(circular markings present on the outer surface) of golden hue arranged like a square. ii) Marked with a shankha and chakra on the lower middle portion. Grants prosperity & fulfilment of all desires on being worshipped.
● Hayagriva: i) Blue in colour at the top, shaped like an elephant-goad or the head of a horse, marked with a linear mark present near the chakra and several bindus. ii) Marked with 5 linear marks, other characteristics being the same before. iii) Marked with a single chakra and a marking resembling a banner. iv) Has both yellow and red colours on its surface, endowed with two chakras, marked with markings resembling ear-rings and gems on the sides, shaped like an elephant-goad or lotus-bud. v) Shaped like a lotus-bud while the mukha shaped like the head of a horse, and marked with a marking resembling a rosary. vi) Green in colour, marked with a single chakra, the left side is elevated like that of a kapinjala bird. Grants knowledge, prosperity and wordly enjoyment on being worshipped.
● Parameshthin: i) Presence of a hole(which does not reaches the other side of the stone, otherwise the stone would be unfit for worship) at the top, has a single chakra, marked with the markings resembling a lotus and several bindus. ii) White in colour, has a single chakra and markings resembling a lotus, shaped like a snake's hood/pearl necklace/sphere, the top is perforated and hard. iii) Reddish in colour, circular in shape with a single chakra and linear mark, perforated and hard. iv) Round in shape, yellow in colour with a hole at the top. v) Red/white in colour while the top surface is yellowish and perforated, marked with a single chakra and markings resembling a lotus. Grants enjoyment & liberation on being worshipped.
● Hiranyagarbha : i) The colour is yellow like that of honey and slightly elongated in shape, has several golden linear marks on the body with a crystal-like glow & shaped like the round moon. ii) Black in colour, round in shape, the vadana is glazed, marked with a single chakra & marking resembling srivatsa(a triangular mark on Vishnu's chest) on a of the top surface. Grants prosperity & increase in progeny on being worshipped.
● Chaturbhuja : The colour is blue like that of a rain cloud. It is round in shape & marked with 4 chakras. Grants fearlessness on being worshipped.
● Gadadhara : Circular in shape, yellow in colour, the chakra is situated towards the left, the middle part is marked with 3 linear marks, markings resembling a banner, a vajra & elephant-goad are present on it.
● Narayana :i) Shyama(dark colour of lighter tone) in complexion, the chakra is situated at middle portion, marked with a long linear mark, the right side is perforated. ii) The vadana is situated at the middle of the body, within which lies the chakra at the front, endowed with markings resembling ornaments like ear-rings, bracelets & nekclaces. Grants all sorts of success on being worshipped.
● Govinda : i) Black in colour, shining in appearance, endowed with a single chakra alongside markings resembling a gada on that right side and that resembling a mountain on the left. ii) Black in colour, not much large in size, the central portion is raised upwards, endowed with a large vadana marked with 5 chakras situated towards the left
● Vishnu : Black in colour, possesses a large chakra, shaped like a gada, a linear mark is present on the top surface near the vadana. Grants liberation on being worshipped.
● Madhusudhana : Golden in colour with shining appearance, has a single chakra & endowed with markings resembling a lotus & a shankha. Destroys enemies on being worshipped.
● Trivikrama : Shyama in colour, triangular in shape woth shining appearance, has 1 or 2 chakras along with some bindus on the left-side & a linear mark on the right-side . Grants wealth on being worshipped.
● Shridhara : i) The shape is as round like the kadamba flower, endowed with vanamala & 5 linear marks at the top surface & sides. ii) Green in colour, round in shape with a flat upper surface, endowed with a single glazed chakra & markings resembling a lotus in the middle. iii) Small in size, has 2 chakras & a vanamala. iv) Glittering like a gem, endowed with a single chakra, vanamala, markings resembling a lotus/banner & another resembling an elephant-goad near the vadana. Grants prosperity to householders on being worshipped.
● Hrishikesha : Shaped like a half moon with a single chakra and also with markings resembling the hair of a boar.
● Padmanabha : Reddish in colour, endowed with a semicircular chakra & markings resembling a lotus & hairs of a mane. Causes poverty & sorrow, hence shouldn't be worshipped.
● Damodara : i) Big in size, the colour is as green as durva grass with a small chakra situated in the middle, a small vadana & a single yellowish linear mark in the middle ii) Endowed with 2 chakras & a cracked outer surface. Grants auspiciousness on being worshipped.
● Sudarshana : i) Green in colour with gliterring appearance, the chakra & markings resembling a gada are situated on the left side while linear markings arranged like a lotus are found on the right side. ii) Black in colour, shaped like a lotus, endowed with a single chakra, a large vadana & lowered middle portion. Destroys all sins & grants results of all types of worship the on being worshipped.
● Vasudeva : White in colour, endowed with a single or 2 chakras at the dwara. Fulfills all desires on being worshipped.
● Pradyumna : i) Yellow in colour, has a small chakra & markings resembling a makara on the sides & top surface, endowed with many holes. ii) The colour is as blue as rain-clouds, has a small chakra & small mukha & many holes. Grants prosperity & happiness to householders on being worshipped.
● Aniruddha : i) Blue in colour, round in shape and glazed, has markings resembling a lotus at the top surface and 3 linear marks near the dvara. ii) Black in colour with a beautifully shaped dwara and having a chakra near the centre, another on the sides and a small chakra at the top surface. iii) Yellow in colour, round in shape. Grants happiness to householders on being worshipped.
● Purushottama : i) Golden in colour with a chakra at the middle portion and a bigger chakra at the front. ii) The colour is as yellow as the atasi flower and endowed with many bindus. iii) With mukhas on all directions (traditionally numbered to ten). Increases prosperity & grants enjoyment and liberation on being worshipped.
● Adhokshaja : Dark black in colour with patches of brown, circular in shape, endowed with a single chakra & red linear marks, maybe large or small in size. Grants auspiciousness to worshippers on being worshipped.
● Achyuta : Has four chakras on right and left sides and 2 red markings resembling ear-rings at the mukha, also marked with markings resembling shankha, gada, bow, arrow, pestle, banner, white umbrella and a red elephant-goad.
● Upendra : Green in colour with glittering appearance, has a glazed body with one or more chakras on the sides.
● Janardana : Has 2 dwaras; one on the eastern side & another on the western side & 4 chakras; 2 on the top surface & other 2 in the middle. Grants prosperity & destroys enemies on being worshipped.
● Lakshmijanardana : The colour is as blue as rain-clouds with one dwara & 4 chakras.
● Hari : Green in colour, round in shape with one mukha at the top, the lower portion is marked with bindus. Fulfills all desires on being worshipped.
● Ananta: i) Marked with markings resembling the hood of a snake, has 20 red chakras, 14 dwaras & many bindus, has different colours on its bod. ii) The colour is as blue as a rain-cloud, circular in shape, has 27 chakras. Grants 4 aims of human life & results of all types of worship on being worshipped.
● Yogeshwara : Has a shivalinga like structure on its top surface. Grants redemption from the great sin of brahmahatya on being worshipped.
● Pundarikaksha : Has markings resembling a pair of eyes/lotuses on the left or top surface or sides. Can bring the entire world under control of the worshipper on being worshipped.
● Chaturmukha : Has 4 linear marks on the sides, and 2 chakras on the middle portion of the body.
● Yajnamurthi : Has both yellow & red colours on its surface, with a small dwara and 2 chakras; one at the middle and other on the right side.
● Dattatreya : i) Has white, red and black patches and markings resembling a rosary on the top surface. (Some versions state red & yellow colours)
● Shishumara : Long in shape, with a deep triangular opening and having one or two chakras on the front side and another on the back side. Grants all sorts of success on being worshipped.
● Hamsa : Shaped like a bow having both blue and white colours on its surface and has a chakra and markings resembling a lotus. Grants only salvation on being worshipped.
● Parahamsa : The colour is blue like the throat of a peacock, with a glazed body and round dwara within which lies a single chakra & a glittering patch resembling the sun on the right side of the chakra. There are also 2 linear marks forming the shape of a boar on the body. Grants 4 aims of human life on being worshipped.
● Lakshmipati : Black in colour while the sides or the front portion is blue like a peacock's throat, has a small chakra & large vadana. Grants prosperity & wealth on being worshipped.
● Garudadhvajalakshmipati : Round in shape, has golden marks resembling horns and hoofs on the body & a smooth chakra with black linear marks on it.
● Batapatrashayin : Round in shape with white, copper-red and blue colours on its surface, has a single vadana in the middle, 4 chakras & 3 bindus, markings resembling a shankha & a lotus lie to the left & right side of the chakra respectively.
● Vishvambhara : Has 20 chakras on the body.
● Vishvarupa : Circular in shape, has 1 or 5 dwaras and many chakras. Bestows children & grandchildren on being worshipped.
● Pitambara : Round like the udder of a cow or a human breast, has a single chakra. Grants happiness on being worshipped.
● Chakrapani : Round in shape and glazed, with a small chakra and many other prints.
● Saptavirashrava : Round in shape with a small chakra and several golden bindus all over the body. Increases all sorts of prosperity on being worshipped.
● Jagadyoni: A single red chakra is present within the dwara. Grants auspiciousness on being worshipped.
● Bahurupin : With multiple mukhas & single chakra alongside markings resembling a shankha present inside. Grants salvation on being worshipped.
● Harihara : Has 4 dwaras & 2 chakras with a shivanabhi(a special form of shivalinga) like structure on its top surface. Grants prosperity & happiness on being worshipped.
● Swayambhu : Blue in colour with a long and big mukha, and having the body encircled by linear marks. Grants only salvation on being worshipped.
● Shivanarayana : Has 2 mukhas & 4 chakras. Destroys wealth, property & progeny, hence shouldn't be worshipped.
● Shankaranarayana : Has a shivanabhi like structure either sideways or left side or right side.
● Pitamaha : Has 4 different dwaras with a chakra in each of them.
● Naramurti : The colour is yellow like the atasi flower with markings resembling a sacred thread on the sides.
● Shesha : Printed with linear marks forming the coiled body of a snake. Although red in colour, it isn't considered inauspicious.
● Pralambaghna : Red in colour with the markings resembling the coiled body and hood of a snake. Causes death, hence shouldn't be worshipped.
● Suryamurti : Has 12 different chakras either on the body surface or inside the dwara. Destroys illnesses on being worshipped.
● Haiheya : Has one mukha and multiple markings resembling hoods, out of which 2 are found on the right side of the dwara, shaped like a lotus petal with a golden mark resembling an arc. Grants all sorts of success on being worshipped.
● Vishnupanjara : Printed with several linear marks.
● Garuda : i) Shaped like a lotus with three marks one above the other the central line being longer, has 4 chakras. ii) Printed with markings resembling a pair of wings and having two, three or four golden linear marks on his body, green, blue or white in colour. Destroys all sins on being worshipped.
● Matsya : i) Elongated in shape, the colour is like gold/bell-metal, and marked with three bindus. ii) Elonhated in shape resembling a fish, green in colour with crystal-like glow, endowed with 2 chakras at the middle portion & 3 bindus. iii) Has 3 elongated dwaras each possessing a chakra while another chakra is situated at the posterior end shaped like the tail of a fish, the right side is shaped like a cart, the left side has a linear mark. iv) Has a long dwara at the right side, endowed with 3 bindus, a single chakra & markings resembling a shankha & lotus v) Shaped like a fish & a has markings resembling an elongated fish. Grants enjoyment, liberation, fulfilment of all desires & auspiciousness on being worshipped.
● Kurma : i) Shaped like a tortoise with the eastern side elevated. ii) Green in colour, round in shape resembling a tortoise, the top surface being elevated and endowed with red markings resembling Kaustubha, 5 circular markings resembling the sun & a chakra. iii) Endowed with structures resembling the feet of a bed on the sides. iv) Endowed with a single chakra, 3 golden bindus & markings resembling a shankha & a lotus. v) Elongated in shape with the mukha extending from left to right side, and endowed with 5 circular markings resembling the sun. vi) Triangular in shape like the inflorescence of a snuhi plant with chakras on both sides. vii) Round in shape resembling a tortoise, has both blue & red colours on its surface, has a long dwara & 2 chakras imprinted sideways at the middle portion of the body. Fulfills all sorts of desires & increases progeny on being worshipped.
● Varaha : i) Blue in colour, big in size, and printed with odd number of chakras & three linear marks. ii) Endowed with even number of chakras, of which at least one is situated on the right side, and vanamala. This last variety is called Lakshmi-Varaha. Grants enjoyment and liberation on being worshipped.
● Narasimha : Has a very large mukha, 2 chakras & linear marks resembling the mane of a lion. Creates detachment in the mind of the worshipper on being worshipped.
● Vamana : i) Round in shape like the kadamba flower, small in size and marked with 5 linear marks. ii) Small in size with glittering appearance, the shape is circular or triangular like the inflorescence of the snuhi flower, has a chakra on both top and bottom surface with markings resembling the Brahminy kite beside the chakra. iii) Not very small in size, has a single prominent glazedchakra at the centre. iv) The colour is yellow like the atasi flower with an elevated top surface, white bindus at the mukha & an indistinct chakra. v) The colour is blue like the rain-cloud, round like the bilva fruit or seeds of the jujube fruit in shape, has 2 chakras, a vanamala & a small mukha. This last variety is called Dadhivamana. Grants fulfilment of all desires, happiness, prosperity; increase in property, progeny & wealth of householders on being worshipped.
● Parashurama : Yellow or black in colour and marked with markings resembling an axe, has a chakra on either left or right side & markings resembling teeth, either at the top surface or on the sides. Prevents untimely death on being worshipped.
● Ramachandra : The colour is green like the durva grass, endowed with a single chakra, markings resembling a staff on the top surface & 2 linear marks on the sides.
● Krishna : i) Black in colour, endowed with a single chakra at the dwara & a vanamala. ii) Has yellow patches, the dwara is situated at the middle, the top surface resembles a tortoise shell, endowed with yellow bindus at the sides. Grants wealth, crops, happiness & redemption from sins on being worshipped.
● Sankarshana : Red in colour, has 2 chakras joined with each other on the top side, the eastern side is perforated. Grants happiness to householders on being worshipped.
● Buddha : Has a very small mukha but without any chakra. It is also called Nivita(hidden) Buddha. Grants the ultimate position(parama pada) on being worshipped.
● Kalki : i) The colour is black like that of a bee, has 6 chakras & markings resembling a sword/dagger over the mukha on the top surface. ii) Shaped like the face of a horse and marked with 3 chakras. Destroys all evil caused by kaliyuga on being worshipped.
More detailed classification is provided in the Meru Tantra & in the book Shalagrama Kosha compiled by S. K. Ramachandra Rao from a manuscript named Shalagrama Pariksha written by a Telegu brahmin named Anupasimha & unpublished sections of the Sritattvanidhi dealing with Shalagrama shilas.
Shalagrama shilas are used as non-anthropomorphic representations of Vishnu alongside Dvaravati shilas, similar to the use of yantra and kalasha in case of Devi & linga and Baneshvara shilas in case of Shiva. The Pranatoshani Tantra  states that worship of all deities can be conducted on a Shalagrama shila. The Puranas unequivocally state that worship of Vishnu done through a Shalagrama shila yields greater merit than that done through an idol.
Unlike idols, Shalagrama shilas can be worshipped in individual residencies alongside temples, can be carried away from one place to another easily & can be worshipped as long as the chakra inside it hasn't been broken or cracked. Apart from the traditional upacharas (articles of worship) whose number varies from 5, 10, 16 to 18, worship of a Shalagrama shila requires only tulasi leaves & water poured from a special conchshell for ablution compared to an idol which requires upacharas of monetary value like robes & jewellery. Hence worship of Shalagrama shila can be afforded by people even in times of hardship & distress as the only mode of worship.
Hindus hailing from South India bedeck idols of Vishnu with a garland made of 108 Shalagrama shilas. Bengali Hindus observe a ceremony called Svastyayana in which 108 or 1008 tulasi leaves anointed with sandalwood paste are offered on a Shalagrama shila with the aim of healing diseases or being delivered from troubles. This can be accompanied by Chandipatha(ritualistic recitation of the Devi Mahatmya) once, thrice or 5 times; worshipping 1 or 4 clay Shivalingas and chanting the mantras of Durga & Vishnu 108 or 1008 times.
The Pranatoshani Tantra states
More glorification has been done in the Skanda Purana & Padma Purana.