Olivine

Characteristics

• Habit - tabular
• Form - orthorhombic
• Hardness - 6 1/2 to 7
• Cleavage - imperfect
• Fracture - conchoidal
• Streak - white
• Lustre - vitreous
• Colour - yellowish-green, yellow, brown, grey, colourless

Applied to any mineral belonging to a solid-solution series in which iron and magnesium substitute freely. well formed crystal are rare. Major component of Earth's upper mantle.

Some minerals do, however, have reliable and distinctive colours. Silicate minerals that contain large amounts of iron are typically dark green or black. These minerals, which often also contain magnesium, are called ferromagnesian minerals; they include olivine, pyroxene, amphibole and biotite mica.

Certain elements may be able to directly replace (substitute for) the normal atoms in the ideal structure - although, for a comfortable fit, the substituting element must have a similar size and charge to the original atom. This phenomenon is called ionic substitution. An example is the substitution of Fe2+ for Mg2+, or Mg2+ for Fe2+, which occurs in the mineral olivine.

Varieties

Fayalite (iron end), Forsterite (magnesium end)

Peridot

Mined for over 3500 years. Name comes from Arabic 'faridat' for gem. Attractive and affordable. Those larger/greener/strong colour/few inclusions (more transparent with better clarity) are of higher value.

Oldest and most famous source was an island in the Red Sea, now part of Egypt. Had various names - St Johns Island / Zabargad / Zebirget / Topazios - suggests the gem was once called topaz even though we now know they are different minerals. The island still has ancient, extensive workings. Thought that Cleopatra's emerald mines may have been confused with this locality. It is likely that many of her 'emeralds' were in fact peridot. The deep crystals within the volcanic rocks are several cms in length and relatively easy to mine by hand. Biggest crystals are up to several hundred carats in weight. Other courses are Parkistan, Arizona, Myanmar, China, Brazil, Norway, Kenya.

Other stones similar in appearance are chysoberyl, green sapphire, tourmaline, some garnets.

Lore: August birthd=stone; 16th wedding anniversary stone; believed to prevent maddness and give wisdom; protect from eveil if worn

Properties:

• shades of yellowish-green, greenish-brown / transparent to translucent
• brittle, can break into conchoidal fractures like glass
• avoid contact with perfume, hairspray, mild acids, sulphuric and hydrocholic acids; avoid rapid heat changes which can cause cracking
• vitreous, oily lustre
• pleochroic - can be hard to see two colours except in larger stones or those deep in colour. When seen it is the same as the basic colour seen eg: pale green & green
• using loupe may see lily-pad inclusions - a central spot (tiny crystal of mineral chromite) surrounded by tension cracks that form a near halo. Good ID feature

Arab Roots of Gemolgy

Some elements from 'Arab roots of gemolgy: Ahmad ibn Yusuf Al Tifaschi's Best Thoughts on the Best of Stones' (c.1240) trans. Samar Najm Abul Huda. The Scarecrow Press, London, 1998.

Peridot

Peridot starts as emerald but due to lack of maturity and lack of heat it becomes softer and inferior in colour and peridot is formed.

Found in the same mine as emerald but it is rarer. At time the book was written [AD 1240] none were found in any mine, the few in the hands of people being small stones recovered from digging old archaeological sites in Alexandria, being remnants of Alexander's treasures.

Comes in dark, light and medium green, of good clarity, translucent.

Does not possess any of the properties of emerald except that it is a beautiful transparent green stone. It improves and strengthens eyesight when one stares at it. [But in the margin of the text is written - 'It is said if peridot is drunk it is useful against leprosy. this is a great benefit that the author had neglected, may God have mercy on his soul'.]