Shell is a mineral with a hardness of 3 out of 10 on the Mohs scale of mineral hardness [?]. These Variously structured gems are made of calcium carbonate, their full chemical compound being CaCO3 and C32H48N2O11.

Shell is the hard, protective, outer layer covering of certain creatures found in saltwater or freshwater, such as mollusks, snails, and turtles. This layer consists of calcium-carbonate and chitin, secreted by the soft tissue mantle of most mollusks. The inner surface of the shell is made from smooth nacre which covers the mantle.

Shell is used as inexpensive material for various purposes in jewelry, for carving shell cameos, ornamental/decorative objects, and utensils.

It displays that attractive pearl-like sheen suitable for making ornamental or usable objects like handles on knives, trays, jewel boxes, spoons, brooches, rings, bracelets and buttons.

Shell-based materials of particular interest to gemologists include nacre (mother-of-pearl), the fossil nacre that is marketed as ammolite, shell cat's-eyes, and shell cameos.

Organic gem material that may be classified as shell include:

Mother-of-Pearl

This is the iridescent inner lining of a shell, formed from regularly secreted layers of aragonite, that have been deposited as single crystals in a 'honeycomb' order. It is also known as nacre, or MOP (mother-of-pearl).

The play of rainbow colors in nacre comes from the light-diffracting properties of its parallel layers of aragonite crystals as they outcrop on the surface of the nacre. This iridescence can be either subtle and directional, as displayed by MOP, or strong and visible from whatever angle you look at it, as shown by the nacre of the abalone.

Commercial sources of MOP include the nacre of bivalve marine creatures such as the gold- or silver-lipped pearl oyster, the black-lipped pearl oyster, and the gastropod trochus shell. The abalone are sources of nacre that display strong iridescence. Abalone is known as paua shell outside the USA, Australia, and the UK.

Paua shell

Paua shell has bright green and blue colored nacre found on the coast of New Zealand. This is the Maori name for the sea snails and gastropod mollusks which belong to the Haliotidae family. But most of today's commercial paua shell has been dyed to further enhance its colour. So, you would do well to check out all paua shell for evidence of dye between its layers of nacre.

Ammolite

Ammolite is the trade name for a Canadian organic gem material, an actual stone made of the crushed remains of 71 million year old fossil ammonites, which are composed of the same material that is in nacre - aragonite. These fossil ammonites are mined from random deposits in southern Alberta, Canada, and from the Rocky Mountains (from Canada to Southwest USA).

The highly iridescent nacre, in these fossil ammonites, though only a very thin organic layer, has not altered much even after all that time in the ground. Their mineral component consisting of 96% aragonite, with traces of strontium, titanium and copper and iron.

Red, green and orange iridescent colours are commonly displayed by ammolite, but not much of blues and violets. Ammolite is often cut with its shale-retained backing to give the material some durability, as its nacre is quite thin. Lower grades of ammolite are also stabilized by impregnating it with plastic material.

It is easy to identify ammolite because of its fossil nacre's shell-like iridescence, and the presence of intersecting fractures that are often filled with fine-grained greyish sediments of shale or reddish-brown ironstone.

Operculum

Cat's-eye effect can be produced from black-lip pearl oyster, when cut suitably, which misnomerly is called cat's-eye shell.

Operculum, or the shell cat's-eye, consists of the small 'door', or operculum, that is used by some univalve molluscs to protect their body parts against predation.
Opercula used for jewellery purposes are derived from a snail-like sea univalve mollusc, the Turbo petholatus. The turbo's operculum consists of a circular low-domed calcitic mass that has a convex green, yellow and brown patterned porcellanous external surface, and a flattish white base that has an incised spiral pattern.

The shell cat's-eye is not chatoyant; it is simply decorated by an external eye-like pattern. The polished surface of the shell cat's-eye displays a reasonably good lustre.

Shell Cameos

Shell cameos are intricately carved items of jewellery and decoration that have been crafted in Italy since the sixteenth century.
Major shells from which shell cameos are carved include the bullmouth or red helmet (Cypraecassis rufa), the black helmet (C. madagascaris) and the pink conch (Strombus gigas).

As each of these shells is formed from layers of fibrous aragonite crystals, of contrasting colour, and differing fibre orientation, the external surface of these shells can be carved back to yield a white subject against a contrasting coloured background. For example: the red helmet shell yields a white subject on red to orange-red background, the black helmet yields a white subject on dark brown to black background, while the pink conch yields a white subject on pink background.

According to the traditional shell carvers of Torre del Greco, shells used for carving shell cameos include:

C. madagascarensis - the 'sardonica', 'sardonyx shell', the black helmet shell of the Atlantic and Indian Oceans, and sea to the north of the Antilles.

C. rufa - the 'corniola' or red or bullmouth helmet shell from Madagascar and Zanzibar.

C. cornuta - the 'orange shell' from the Indian Ocean.

S. gigas - the pink conch from the Caribbean.

Cypraea tigris and Porcellina tigrina - cowrie shells from the South China Sea.

Tortoiseshell

Tortoiseshell is not actually a shell but a horny, organic substance that covers the skeleton of the hawksbill turtle. The dorsal surface or carapace of the hawksbill turtle is made up of 13 plates, or schutes, each of which is composed of keratin. Keratin is a fibrous protein similar to that forming the horns and hooves of cattle, the fingernails of humans, and the claws of animals and birds. These plates of tortoiseshell have rich, warm, yellow, translucent colour and are attractively marbled and spotted with spots and patches of contrasting reddish, chestnut-brown colour. The ventral surface, or plastron of the hawksbill turtle, is covered by 12 plates that have a uniformly pale translucent amber-yellow colour and show no mottling or streaks.

This tortoiseshell is termed blond tortoiseshell or yellow-belly. It is the finest, most highly prized and valued form of tortoiseshell.

Common tortoiseshell has a translucent yellowish body colour that is attractively patterned by irregularly dispersed reddish brown patches that are formed from aggregates of individual rounded particles of the blackish brown biological pigment, melanin.

Today, tortoiseshell is little exploited, for like all sea turtles, the hawksbill turtle is now a protected animal that is on the edge of extinction.

When the hawksbill turtle was declared an endangered species in 1975, international trading in post-1975 tortoiseshell was declared to be both illegal and ecologically unacceptable.

The specific gravity [?] for Shell is 1.3, it's refractive index [?] is 1.53-1.59, and it's double refraction [?] is None.