JADEITE

A pyroxene and a silicate of sodium and aluminium. It is usually green, black, blue, red, white or yellow. Pyroxenes have a specific chemical formula (ABSi2O6 where A can be Ca, Na, Mg or Fe and B can be Mg, Fe, Cr, Mn or Al). Other pyroxenes include augite, diopside, and enstatite. It is one of two sources of jade the other being nephrite which is not a pyroxene. In the 1800's it was discovered that Jade consisted of Jadeite and Nephrite although previously it was thought to be made of only one mineral. The two minerals are similar in appearance but can be differentiated because Jadeite has a higher specific gravity (weight in comparison with the equivalent volume of water) and is usually a deeper green colour. Jadeite jade is rare and therefore more valuable.

It is fairly hard (7 on the mohs scale) and twinning may be evident. Green gem quality jadeite is known as imperial jade and derives it's colour from chromium impurities. Imperial jade is rare and difficult to work with due to its compact structure but is used particularly in the far East. Jadeite has been used in China since the 1700's for ornaments and was sometimes used in house foundations became it was believed to prevent buildings being struck by lightning. Most Chinese imperial jade was mined in Burma. Lower quality jade is often used for beads and ornaments. Bowenite is sometimes passed off as jade because it is much softer and more workable and can be stained to look like it and increase its value.

It is found in igneous and metamorphic rocks and alluvial deposits. Weathering may create a brown surface coating of iron oxides on some specimens. Best specimens are from Burma as well as China, Japan, Mexico, New Zealand and Tibet.

JASPER

A type of chalcedony (a form of quartz and memebr of the oxide group) which is very variable in colour. Chalcedony is a form of silica which often takes on the colours of surrounding minerals. Jasper is usually red but may also be grey, yellow or black depending on the associated minerals close by. Red Jasper derives it's colour from hematite which is an iron oxide. Grey and yellow jaspers are often influenced by nearby clay deposits whereas deep brown jaspers are sometimes coloured by goethite (another iron oxide). Jasper can change its colour because it is porous and allows impurities to infiltrate although they cannot change it;s chemical composition. It contains the same chemical elements as carnelian and chrysoprase.

It is fairly hard (7 on the mohs scale) and it's colour is often not uniform throughout. Some specimens known as riband jasper have stripes along their surface and are often used for carvings or brooches. It may also be cut into cabochons or used for sculptures, pots, boxes or chess sets.

Black jasper can be used to test the amount of gold in alloys. When an alloy is rubbed along jasper the streak colour can be used to calculate the amount of pure gold it contains.

It forms in veins in sedimentary and metamorphic rocks. It can be found in India, Venezuela and California (US). Red Jasper can be found in France, Germany, Russia, Cyprus, Egypt, Italy and South Africa.