How to judge Peridot

Ancient Egyptians knew it as topazion, in reference to its then only known source on the Isle of Topazos. The Pharaohs had mined the island for hundreds of years from as early as 1500 BC. Today, the isle is known as Zabargad, Arabic for "peridot". Previously known as St John's Island, Zabargad is located 300km east of Aswan, Egypt. It is still a deserted island, but the mines of Zabargad no longer produce the stones.

The green stone Peridot has often been mistaken for emerald because of their physical similarity. The birthstone of August, it is a member of the Olivine family. The variation in magnesium and iron content in peridot may cause it to vary slightly in physical and optical values. Olivine may have originated from the Latin olivia (olive oil) because of its similar colour. It is also referred to as chrysolite (yellow stone in Greek).

Burma is a reputable source for peridot. The quality varies, but emerald-like stones, especially from the Mogok Stone Tract, are exceptional. The San Carlos Apache Indian Reserve in Arizona, USA, is also a good source but large size specimens (1 to 5cts) are difficult to find. In Hawaii they occur in volcanic bombs. In the 1990s, Pakistan became another source and the quality of the stones was similar to the Burmese material. Peridot is also mined in Australia, Brazil, China, Kenya, Mexico, Norway, Sri Lanka, Tanzania and Vietnam.

Colour: The stones are typically yellowish green in differing intensity: bright lime green, brownish-green to dark brown. Cat's eye and asterism are known but they are extremely rare.

Clarity: Peridot is not an expensive stone so stones with good clarity should be the choice. Some stones may look dull and lifeless due to small platelet-like inclusions or strong birefringence. Doubling of back facets is easily seen in larger stones if properly cut. Small inclusions resembling the leaves of water lilies (lilipad) with a tiny negative crystal (chromite) or cavity at the centre are a positive identification. Biotite flakes are common, while tiny spheres of glass are unique to Hawaiian peridot.

Cut: The stones are usually faceted as mixed cut, step cut, cabochon, beads or tumbled. New innovative cutting styles are becoming common because they are affordable for fashion. Poorly cut stones should be avoided. Sizes range from melee to more than 10cts.

Treatments: The stones are usually not enhanced unless mistaken or misrepresented.

Natural Look-alike Gemstones & Simulants: A number of natural stones such as green sapphire, chrome diopside, tourmaline, enstatite and green garnet can be mistaken as peridots. Common man-made imitation such as glass is easily identified by its single refractive property. No synthetic peridots have been reported.

Handling: Peridot flaws readily when dropped or carelessly handled, and is vulnerable to thermal shock as well as chemical attack. It's a relatively soft stone (61/2-7 on Moh's scale). Many stones have been damaged due to negligence during gem setting or jewellery repair. The stone may also lose its polish when exposed to hydrochloric or sulphuric acid. Bear this in mind when sending your peridot jewellery for repair.With green currently in the fashion radar, there's no better time than now to don August's birthstone.