Peridot

Group: Olivine Mineral class: Silicates Colour green-yellow, olive, green, brownish green Colouring substance: iron, magnesium Pleochroism: very weak: green tones or colourless Mohs’ hardness: 6.5 - 7 Density: 3.27 - 4.20 Cleavage: indistinct Fracture: conchoidal Crystal system: rhombic, short prisms Transparency: transparent Lustre: vitreous lustre, oily lustre Fluorescence: none

Peridot is the best variant of the Olivine. Before the times of Alexander the Great it was almost unknown and it only started to be appreciated in classic Greece and Rome. The crusaders took it to Western Europe. In the Middle Ages it was used for religious decoration. In the Baroque it was the favourite stone for rich and noble people.

The best Peridots have 10-15% of iron and 85-90% of magnesium. Due to its high Mohs’ hardness it is easy to cut and polish. However, this gemstone is very sensitive to extreme temperatures or to contact with some acids. Also worth mentioning are the typical inclusions of dark grains of chromite (chrome oxide and iron) surrounded by small fractures, which can only be observed under a microscope.

It is found among layers of basalt and other dark volcanic coats. In the Red Sea (the island Zebirget – St. John), Burma, Bohemia (this was, until the discovery of the deposits in Zebirget at the beginning of the 20th century, the most important source), Brazil, Australia, Norway, Germany, USA, Mexico and Lanzarote (Canary Islands).

Peridots are very versatile. Generally they are very faceted oval, brilliant or circular cut.

Beryl, tourmaline and some emeralds can be similar. There are also plenty of crystal imitations in the market. A good way to distinguish a peridot is looking at the deviation of the light inside of the stone, which is especially apparent in big stones: it makes the gemstone look a lot more faceted than it really is.