A Background to Shell

A BACKGROUND TO SHELL

A shell is the hard part of the body of animals called molluscs. It is thought that there are over 100,000 different species of mollusc, some living on land and some in water, both fresh and marine. The phylum (primary division of the animal kingdom) ‘mollusca’ includes – amongst others -- oysters, mussels, garden snails, cuttle-fish and octopuses.

Molluscs are invertebrates – that is, they have no internal skeleton. They are soft-bodied, cold-blooded, and usually without limbs. Not all of them have shells, and some of them have developed so that their ‘shell’ is inside the body.

A collection of marine mollusc shells showing their variety.

Some molluscs are highly prized because they produce pearls, but many more are esteemed for their decorative shells which come in a huge variety of colours, shapes and sizes, from the tiny ones found on any beach to the giant clam shells which are large enough to be used as church fonts.

Jewellers and antiques dealers generally come across just a few types of shell in the course of their work. Living ‘bivalves’ have two shells joined together, but are very seldom encountered this way after they have been killed. They include the well-known pearl-producing varieties such as the marine Pinctada oysters and the fresh water Hyriopsis mussels. Gastropods have one shell and include abalone, New Zealand paua, and ‘ear shells’ -- all from the family Haliotis – and queen conch (Strombus gigas), and the various ‘helmet shells’ of the family Cassis. Top, turban and cowrie shells are also Gastropods, as indeed are garden snails. Nautilus – a very ancient creature whose shell has long been popular in the decorative arts -- belong to the Cephalopod group.

It is very likely that today shells are being used from groups other than those mentioned above for inexpensive jewellery such as beads, but it is seldom possible to identify them when they have been cut, polished and possibly dyed.

Bride price shell bracelet, from Papua New Guinea

THE USES OF SHELL

To primitive man molluscs were food, and having eaten the soft body of the animal, he could use the empty shell as a shovel, or as a container, or a drinking vessel. Small shells could be threaded on a string as jewellery, and large or broken shells could be used for cutting purposes. Today large shells are cut and the pieces polished into spheres to be used as beads. The inner, 'mother-of-pearl' layer of some shells has been used for centuries for everything from furniture inlay to knife handles, or carved and used for brooches, fans, and such like.

Left: tiny 'Maireener' shells, used by Tasmanian Aboriginal women, and right: young live abalone farmed in Thailand for meat and shells.

Today molluscs are still eaten. Oysters are regarded as gourmet food, while abalone are farmed for their meat which is eaten as a delicacy or made into a sauce, and their shells then turned into jewellery or other decoration.

Shell has been appreciated and used world-wide. In some countries shells have been used as currency, in others as decoration on houses, whilst in other places ceremonial clothes --everything from hats to horses’ bridles – have been adorned with shells. The modern shell-encrusted souvenir from a seaside resort is not a new idea but stems from the art of producing magnificent displays and arrangements – such as those simulating vases of flowers or framed pictures that were popular in the nineteenth century. These items were commonly the work of whalers.

Shells are made of calcium carbonate crystals (in the form of aragonite or calcite) held together by an organic matrix – a substance made of fibrous proteins -- called conchiolin. Shell can be cut, carved, dyed, polished, and made into laminates, or shells can be used simply as ornamentation in their own right, and admired for their delicate and intricate patterns and colours.

Left: shell jewellery of natural colour, and right: dyed shell jewellery.

For decorative purposes, the shells used can be divided into two categories: those with a nacreous surface (a ‘mother-of-pearl’ surface which defracts and reflects light giving an impression of a rainbow of colours), and those with a porcelaneous surface (a soft sheen resembling porcelain).

The nacreous shells, which include pearl-producing molluscs such as the silver-, gold- and black-lipped oysters, and abalone (or paua) shells, are put to decorative uses where little needs to be done to the material other than shaping it, for example to be set in jewellery. The surface is sufficiently beautiful for it to need little treatment other than an extra polish, though it is sometimes dyed to alter or enhance its colour, or covered with a polymer cap or coating to protect it.

Abalone shells, some polished on the outside revealing interior

and cabochons dyed and capped with clear plastic.

On occasion a nacreous shell is polished from the outside, where the outer dull layer is removed to reveal the ‘mother-of-pearl’ layer beneath. This happens when the whole shell is being used as some form of decoration.

A much more mundane use for nacreous shells is the button industry. Any shell can be used, but commonly the trochus (top shell) was the favourite. Mother-of-pearl buttons can be identified by examining the back of the button, where remnants of the shell are usually visible. In the case of the common, white buttons from trochus shells, the reverse displays patches of pink, brown, or green. Before the advent of plastics, mother-of-pearl buttons were plentiful. Today most of the ‘pearl’ buttons are plastic imitations, though buttons made from natural material are still available in specialist shops.

Top shells, used for the button industry: with button

blanks cut out, whole shells, and buttons.

The porcelaneous shells are seldom left unworked. They may be cut into spheres and used as nuclei for cultured pearls, or dyed as beads or pearl imitations. Two types -- helmet and queen conch shells -- need intricate work to bring out their beauty. These shells have a structure consisting of different coloured layers, and when they are carefully carved the result is a cameo, where the dark background (the inner layer of the shell) contrasts with the pale outer layer.

Whole helmet shell, with outer creamy layer intricately carved

to reveal the darker layer beneath as a cameo.

When identifying shell cameos it is important to look at them under magnification. This will show that shell cameos display very fine striations, which are totally absent in hardstone cameos (those carved from, for example, onyx), or plastic imitations. If the back of the cameo is visible it will usually show a little of the natural curvature of the shell, though this may be difficult to discern in very small items.

Left: cameos from helmet shells, and right: detail of cameo showing minute striations in the structure of the shell.

Shell can also be used to simulate other materials, e.g. large spheres shell are used to imitate South Sea pearls and Corallium coral, by giving them a coating of the appropriate colour. They cost a fraction of the price of the objects they are imitating. (See ‘A Background to Pearls’, and ‘A Background to Coral’.) The effect can be convincing, but in the case of South Sea pearl imitations the finished shell product is altogether too perfect in appearance. A string of ‘pearls’ may be too well matched, the sizes all exactly the same, the colours too identical, and the ‘pearls’ themselves too round and free of any kind of blemish. As the large, thick shell types used often form with distinct layers of nacreous and non-nacreous material, it is often possible to glimpse the layers beneath the coating when a light is shone through a shell bead.

Left: coated shell beads imitating South Sea pearls; centre: coated shell beads imitating

coral but displaying structural layers of shell, and right: shell beads, showing layered structure.

Today there is a large market in shell pieces, off-cuts and chips which have been tumbled in order to round the edges (though not necessarily to make them into any particular shape), and frequently dyed. In this way no shell bi-products need go to waste, and even the most unattractive and inferior shells can be made into something marketable. An example of this is the pieces of trochus (top shells) which remain after button blanks have been cut away. These mother-of-pearl strips make pleasing and striking beads of slightly spiky-looking shapes.

Another modern use of shell is as laminate. Very thin slices are cut through the shell and backed with self-adhesive material, while the visible surface is coated with a polymer to give it body. Shell chips can also be embedded in polymer and moulded.

Paua shell laminate, and moulded plastic kiwi souvenir

containing chips of paua shell.

An indication of plastic is discolouration with age. Plastics may turn yellow, while white shell retains its colour. However, some shells do fade with age, for example the queen conch, the pink colour of which finally fades to white. Again, it is important to examine a white cameo under magnification and look for the tell-tale striations of shell material. If these are absent, the cameo is probably made from hardstone, though if the material is light and warm to the touch (as opposed to cold – a characteristic of both shell and hardstone), it could be plastic.

Top: penknife with discoloured plastic imitation

mother-of-pearl, and bottom: real mother-of-pearl.

Amongst the Gastropods there are some shells that have a ‘door’ that they can close over the shell’s opening to keep out predators. This is called an operculum. In most cases it is made up of a horny substance, but in a few cases it is calcareous and suitable for use in jewellery or as ornamentation. Usually greenish brown on a white background, the most common opercula seen in the antiques or jewellery trade comes from the green turban shell.

Opercula from turban shells, left: reverse side; centre and right: outside.

The nautilus is an animal that has changed little over the past several million years. It is unmistakeable in shape, or in its colouring of brown and white stripes. It lives at depth in warm oceans, and its thin shell with a nacreous lining was used as inlay before the advent of laminates. Much admired for their shape, whole shells have also been polished, carved and etched, and used simply as decorative items. The central whorl of the shell is a steely blue colour and can be cut out and mounted as a large, pearly cabochon – often erroneously thought to be a cultivated blister pearl. (See ‘A Background to Pearls’.) It is called a ‘coque-de-perle.’ Blister pearls are seldom oval in shape and never have the very slight ridging that can be seen across the surface of the coque-de-perle.

Left: sectioned nautilus shell showing interior 'septas' and mother-of-pearl lining;

centre: coque-de-perle showing ridges (the rule is in centimetres), and right: inexpensive jewellery using endangered baby nautilus.

Ancient relatives of the nautilus were ammonites. These are found as fossils, and are cut in half, sometimes displaying attractive brown colours and beautifully crystallised centres. They have in the past been sliced and used as inlay, but today are regarded more as collectors’ items.

A species of ammonite that is found exclusively in Alberta, Canada, is sold as ‘ammolite.’ In this material the process of fossilisation has not destroyed the aragonite crystals on the outermost surface of the shell, and these produce a magnificent iridescence. It is an expensive gem material, similar in appearance to some opals. The highest quality is only lightly polished on the surface before being mounted in jewellery. Material of lesser quality may be backed to give it more body, whilst the poorest quality may be backed and capped with a clear polymer to make it more stable.

'Ammolite' pieces, capped with plastic for durability.

It has already been mentioned that some shells are capable of producing pearls. The shells can be of varying types and shapes, but the best known are the nacreous shells specially cultivated for their pearl producing properties. The abalone is cultivated for its shell and meat but can produce both (very oddly shaped) natural pearls and cultured blister pearls. The latter must be cultivated with the utmost care, and it is impossible to culture full pearls from abalones because the animals are ‘haemophiliacs’ and would die. A few porcelaneous shells occasionally produce pearls, and these all occur naturally. Of these can be named the clam (though clam pearls tend to be dull and of poor colour), the conch, which can produce beautiful little pink pearls – often oval in shape -- with a ‘flame pattern’ on the surface, and the baler or volute shell which can produce a very rare orange pearl with a similar flame pattern, called a melo pearl. (See ‘A Background to Pearls’.)

SHELLS AND CONSERVATION

Most shell available on the market today is the product of farming and the cultivation of pearls. It is worth noting, however, that some species of mollusc are protected under international agreements such as CITES (The Convention on International Trade in Endangered Species of Wild Fauna and Flora.) One such example is the freshwater pearl-producing mussel which can be found in Scottish rivers. These have been fished almost to extinction, so the practice is now banned. The same applies to some of the mussels in American rivers.

Nautilus are not yet protected. This is because they live at such depths that very little is known about them, and, without the data, no official ban can be declared. It is known, however, that they are fast becoming endangered, and so use of these shells – especially the small, immature ones that are popular in inexpensive jewellery – should be discouraged.

Generally, shells found whilst beach-combing can be removed from the beach, but this is not always the case and it is advisable to check with local authorities if in any doubt. An example is the queen conch, which is listed by CITES and is thus subject to a fishing quota.

It is also sensible to bear in mind that a few molluscs contain a chemical agent as their protection against predators. This may be in the form of poison or as a dye. Care should therefore be taken when collecting shells if the mollusc is found to be still alive.