NACRE

An organic substance with a variable chemical composition and no crystal structure (amorphous). It forms as part of the shell of some molluscs including pearl oysters. It often contains aragonite, which is form of calcium carbonate and a member of the carbonate group of minerals. Although nacre often has an identical chemical composition to aragonite, it cannot be classed as a carbonate because one specimen may differ greatly from another. It is very soft (3.5-4 on the mohs scale) and derives its name from the Arabic word naqqarah, which means shell.

Nacre is formed by oysters and mussels when a foreign body is present within their shells e.g. a parasite or grain of sand. Sheet like aggregates of calcium carbonate crystals grow around the foreign body in layers on top of each other. These layers are bound by conchiolin and water. Light passing through the layers creates a beautiful play of colour on the surface of the most precious specimens (known as iridescence). It is also known as mother of pearl because it is precedes pearl, which is usually round and more developed. Nacre forms in thin sheets around the outer edge of the shell but is less valuable than pearl.

Pearl and Nacre both contain water and are classified as hydrated gemstones. If they dry out their surfaces may crack and they will become less resistant to erosion. Therefore they should never be stored in very dry atmospheres and should be wiped occasionally with a damp cloth to keep them looking their best.

The finest nacre forms in the shells of pearl oysters which can be found in sea and fresh water. They can be found along the Great Barrier Reef off the coast of Queensland, Australia, in the Persian Gulf, the Red Sea, the Gulfs of Mexico and Panama and in the waters off Venezuela.

NEPHRITE

A silicate which is usually dark green in colour due to the large quantities of iron it contains. The higher the concentration of iron, the richer the green colour. Sometimes the iron may be partly substituted by magnesium (magnesium rich specimens may appear white). It is one of two sources of jade along with jadeite, which is more valuable and less wide spread. It tends to form in aggregates which interlock tightly making it very resilient and hard-wearing. However, it is softer (6-6.5 on the mohs scale) and less resistant to wear and tear than jadeite. This along with its inferior colouration limits its use in jewelley. It derives its name from the Greek word nephros, meaning kidney, because during the middle ages it was thought to be beneficial in the treatment of renal disorders.

Jade was thought to be made up of a single mineral until 1863 when it was discovered that it could be derived from nephrite or jadeite. These two minerals can be distinguished since nephrite has a splintery fracture and an oily sheen. Also jadeite has a more granular appearance, whereas nephrites texture is more like felt. Although it is fairly hard wearing, it has a less compact structure than jadeite which means that specimens may chip and crumble while being worked. However, the finest nephrite is used for the same purposes as jade such as jewellery and decorative objects. A famous ornament of a snail was carved by a Russian jeweller Peter Carl Faberge (1846-1920) with the naturally greasy appearance of the mineral creating the slimy appearance of the snail.

Jade carved from the 18ths century onwards tends to be made of jadeite, whereas older pieces are more likely to be made of nephrite. The Chinese have carved nephrite for more than 2000 years, although the raw materials were probably imported from Turkestan in central Asia and later from Myanmar. It was also used by the Maoris in New Zealand for their sacred carvings.

Nephrite may sometimes be confused for bowenite because they are very similar in appearance. It can be imitated by aventurine and vesuvianite and can even be simulated by a form of paste which is recrystallised, although this is less reflective of the incoming light and therefore less sparkly than the real thing. Nephrites natural colour can be enhanced with dyes.

It forms in metamorphic rocks. The finest specimens come from China as well as in British Columbia (Canada), New Zealand, Poland, Siberia (Russia) and in Alaska and Wyoming, US.