OPAL

Opal is a variety of quartz and therefore an oxide of silicon. It is really a hardened silica gel which has some water attached to it (6-10% of its whole mass). It is fairly soft for a gemstone (6 on the mohs scale) and is amorphous, which means it has no crystal structure and can take almost any form. Opals are transparent and have a glassy sheen. They can be found in many different colours including milky white, black, red, blue and yellow. The colour an opal appears depends on its internal structure. They are made up of spheres of silica which diffract light to give off beautiful colours (an effect known as iridescence). The larger the spheres the larger the range of colours. Sometimes holding an opal in the palm of your hand can expand the spheres and increase the colour range. It is also thought that colour can be influenced by the size of the gaps between spheres. The more striking the interplay of colours the more valuable the gemstone. Non precious opals are often completely opaque and show no colour at all.

Opals are very fragile when exposed to the atmosphere because they can lose water molecules and develop tiny fractures. When heated they can lose all of their water and may turn into chalcedony or quartz. They are mainly used for jewellery and can sometimes be more be expensive than diamonds. It is important that they are cut by a skilled professional since they are very soft, fragile and sensitive to heat and need to be set in a particular way in order to protect them against wear and tear. Some precious opals are used in industry as abrasives and in ceramics (especially black opals from the Australian desert and Mexican Fire Opals).

There are many different varieties including fire opal (red and yellow and so named because it produces reflections that resemble flames), wood opal (in which wood has been replaced partly by opaline silica), geyserite (found around hot spring geysers) and boulder opal (brown and found in sandstone veins).

Opals are most often found in sedimentary rocks that have formed around silica rich waters. They may appear in large lumps or in tiny grains and can sometimes replace organic material such as wood and coral (e.g.wood opal). They can be found in a wide range of locations including Romania and Idaho and Nevada (US). Black opals are usually from New South Wales (Australia) and fire opals from Mexico. Geyserite can be found around hot water springs in Iceland, New Zealand and Yellowstone Park (Wyoming, US). In UK opals have been found in Devon, Cornwall and the Lake district.

PERIODOT

A silicate containing magnesium and iron with silicon and oxygen. It is the gemstone variety of olivene and is usually transparent or translucent with a glassy or greasy sheen. It is green due to the presence of iron which is an essential part of the gemstones makeup and not an impurity. The more iron it contains the greener it will appear. Specimens with less iron contain more magnesium which may give the them a yellow colouring. The greater the concentration of magnesium the more yellow the gemstone. Yellow periodt is sometimes known as chrysolite. One characteristic of Peridot is that it reflects light so that the facets at the back on the gemstone can be seen from the front. This effect is known as double refraction or birefringence. It is moderately hard (6.5-7 on the mohs scale) and can sometimes be mistaken for chrysoberyl.

Large pieces are rare and may be very expensive. Most gem quality specimens are small and are often polished and made into necklaces or cut into cabochacons. Many samples have been preserved from Roman times and can be seen in museums worldwide. For example, the Smithsonian museum in Washington DC (US) has two particularly fine green stones originally found in Myanamar. Peridot's value has risen and fallen dramatically through the ages. Today is has become less valuable although this may be just to do with fashion.

Periodot belongs to the olivene family of minerals which make up the forsterite-fayalite series. Forsterite is a silicate of pure magnesium and fayalite is a silicate of pure iron. The minerals in between are classified according to their iron content. Peridot contains roughly equal concentrations of magnesium and iron and lies roughly in the centre of the scale. Impure peridot samples tend to contain more magnesium. Some peridot is known as forsterite olivene because it lies towards this end of the series.

Most peridot occurs as inclusions in basalt and gabbro (igneous rocks). Many of these formations have been moved closer to the earths surface by erosion and folding. If they had remained in their original locations of formation they may have been too deep fro extraction. It may also be formed my metamorphism and can be found in some sedimentary rocks and in some iron based meteorites. It has been mined for at least 3500 years with the oldest source being the deposit on the Egyptian island of Jazirat Zaberjab in the Red Sea. It was from here that the crusaders first brought peridot to Europe in the 12th century. It can also be found in Australia, Brazil, Kenya, Italy, Mexico, Canary Islands (Lanzarote) and Ross Island (Antarctica). Some peridot has been extracted in Siberia, Russia from a crashed meteorite which was discovered in 1776.