IRIS QUARTZ

A rare form of quartz and a member of the oxide group. Although it is colourless it's internal crystal structure results in the diffraction of light into all colours of the rainbow. This property was first discovered in Roman times. It is named after the word iridescence (also know as spectral diffraction) which is itself a word derived from Iris, the Greek god of the rainbow. Iridescence occurs when white light passes into a solid medium which then breaks up the rays into different colours. In Iris quartz this is caused by fractures inside the gemstone. Filling the gaps with particles of liquid or gas may enhance the colour effects. Labradorite has a similar property which may be enhanced by cutting.

It is relatively hard (7 on the mohs scale) and is used in jewellery and for ornaments or statues.

It is mainly found in pegmatites. The internal structural flaws are the result of mechanical pressure at the time of formation. It may be created by heating and then rapidly cooling ordinary quartz. However,the quality of iris quartz manufactured in such a way will diminish with time.

It can be found in Brazil, Italy, Madagascar, Portugal, Russia and Switzerland.

IRONSTONE

Ironstones are a group of iron rich sedimentary rocks which are largely composed of hematite but have a very variable composition. They usually contains iron, hematite, limonite, magnetite and some silica. Iron bearing minerals are often cemented together by calcite and dolomite. They are usually grey and metallic in appearance when extracted but become red or brown when they come into contact with the atmosphere (rusting).

In order to be of gemstone quality minerals must appear in parallel zones which is often rare. They may sometimes contain alternating layers of pink quartz and hematite. Although they are more difficult to cut and shape than hematite, some ironstones may be made into decorative slabs. If they are very rich in iron (at least 15%) this may be extracted for use in industry.

Ironstones have formed from the Cambrian period onwards and still form today in tropical lakes and swamps. They are usually classed as sedimentary rocks rather than gemstones and are found with cherts, limestones and sandstones. Some may contain fossils.

Ironstone can be found in Australia, Lake Superior (Canada/US) and in England and Wales..