MARBLE

Although marble is considered a gemstone it is really a metamorphic rock formation. It is difficult to define due to its variable chemical composition but its principal component is usually calcite (naturally occurring calcium carbonate). It may also contain diopside (a silicate of magnesium and calcium), albite, apatite, biotite, chlorite, tourmaline, vesuvianite and many other minerals. Some fossils may be evident. The finest marble is white but it may also be black, red, green or grey.

The name marble is derived from the Greek word marmairein which means to gleam and refers to its distinctive sheen (lustre). It is used in the construction industry and for monuments, statues, table tops and ornaments. Marble for statues must be pure white with grains of uniform size. Although it is fairly soft (2.5-3 on the mohs scale) it is used for jewellery. Gem quality marble must have a consistent colour and be resistant to abrasion. The most famous and valuable form is Carrar marble from Italy. (a creamy variety of marble used worldwide for sculptures).

It is formed by the alteration of sedimentary or metamorphic rock and is therefore classed as metamorphic. It is often found in association with other metamorphised sediments such as quartzite and schist. It can be found in Algeria, Austria, Greece, India, Ireland, Italy, Norway, California USA.

MOON STONE

Moonstone is the name given to iridescence pieces of light blue orthoclase feldspar. It is a silicate of potassium and aluminium as well as silicon and oxygen. It has perfect cleavage and crystals often run into each other (twinning). It is relatively hard (6-6.5 on the mohs scale) and is often transparent and pale grey or brown in colour. However, the finest specimens have a blue or white lustre which reminded people in ancient times of the shining moon (hence its name).

The most distinguishing feature of moonstones is their sheen which varies from specimen to specimen. Some show a play of all the colours of spectrum on their surface, an effect known as iridescence (after Iris, the Greek god of the rainbow). Some specimens have a sheen which makes them resemble opal, an effect known as opalescence. However, the rarest moonstones have an unusual metallic lustre known as schiller (from the old German word meaning to twinkle). The schiller effect results when thick layers within the rock create a shimmering play of light on the surface (iridescence). These beautiful optical effects occur due to the reflection of light within the rocks internal structure. Moonstones contain alternating layers of albite and orthoclase. A thin layer of albite produces a distinctive blue colour. Thicker layers result in a white schiller effect.

Large workable specimens are rare which increases their value but restricts its practical use in jewellery and ornamentation. Much moonstone is also flawed which further reduces its suitability for jewellery. It is also highly susceptible to decomposition. In ancient times it was popular with moon worshippers although some mineralogists believe that what early civilizations called moonstone was in fact selenite, the transparent form of gypsum. As it is not clearly defined any mineral can be described as moonstone if it has particular optical properties (blue colour and distinctive surface play of light). It is also known as adularia, which is derived from the Adula mountains in Gotthard, Switzerland.

Like all forms of orthoclase it can be found in igneous, metamorphic or sedimentary rocks and usually in aggregates (massive formations). Its most prominent source is in the gravels and pegmatites of Sri Lankas Dumbara district. It can also be found in Brazil, India, Italy, Madagascar, Mexico, Burma, Tanzania and US.