Quartz
Group: Quartz
Hardness:7 Mohs
Specificgravity:2.65
Properties: Crystalline; asterism and chatoyancy
The quartz group includes amethyst, citrine, rock crystal, tigereye, and rose, smoky and rutilated quartz. Stones are susceptible to wear as they are only moderately hard, but this makes them ideal for carving and engraving.
Range available: Relatively inexpensive. Good-quality material is usually faceted, with lower-grade crystals being cut as beads or cabochons. Tigereye quartz is found in cabochon form.
Jade
Group: Jade Hardness: 6.5-7 Mohs Specific gravity: 2.96-3.33 Properties: Greasy to pearly lustre
There are two types of j’ade – jadeite, which is a precious gem usually coloured green but also found in a wide range of pastei shades, and nephrite Jade, which is spinach- or sage-green and ideal for carving.
Range available: Both jadeite and nephrite Jade are used for cabochons, beads and carvings. Bright green imperial Jade and lavender Jade can be expensive if the colour is pure and the material is opaque. Dyed Jade will be much less valuable.
Agate
Group: Chalcedony/agate Hardness: 6.5-7 Mohs Specific gravity: 2.58-2.64
Properties: Fibrous, porous aggregates, waxy dull lustre
Agate often contains banding or dendritic inclusions within the microcrystalline structure of the quartz. The base material is translucent, with various mineral deposits causing the effects.Perfect to make a nice piece of fashion jewelry
Range available: Banded, moss and dendritic agates are cut to show the inclusions or colours to the best advantage, often as cabochons or slices available in a wide varietyofshapes, including freeform. The more beautiful the effect in a stone, the higher the price will be.
Peridot
Group: Peridot Hardness: 6.5-7 Mohs Specific gravity: 3.27-3.37
Properties: Distinct striations along the lengths of the crystals
Peridot is also called olivine, which describes its colour. It often contains inclusions of silica, mica or spinel. Polishing with the stone in situ should be avoided, as peridot is quite sensitive to heat, as well as chemicals. Stones are often step or mixed cut to reduce the risk of breakage.
Range available: Paler colours of peridot are inexpensive; lower-grade material is used for cabochons and beads. Large stones with few inclusions will carry a high price and are often used for fashion jewelry.
Opal
Group: Opal Hardness: 5.5-6.5 Mohs Specific gravity: 1.98-2.50 Properties: Iridescent
Opals are a composition of hydrated silica gel with a water content of 5-30 per cent, and are generally described bythe background colour of the stone, which ranges through white, black, orange or red (termed fire opals). Vibrant iridescent flashes of colour give opals a unique quality.
Range available: A wide variety of colour combinations of opal are available, usually cut as cabochons. Because opal is such a temperamental stone to work with, it is often supplied as ‘doublets’ or ‘triplets’. These composite stones have a layer of common opal set under a layer of precious opal. Triplets also have a protective top layer of rock crystal.
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