ARTICLE ON SPINEL - A GEMSTONE

As part of IDL education program to the trade about gemstones and jewelry, here is a new article on one of the most beautiful gemstones available in the market.

The stone described is the Spinel.

Spinel is the magnesium aluminum member of the larger Spinel group of minerals.

It has the formula MgAl2O4.

It crystallize in the cubic (isometric) crystal system, with the oxide anions arranged in a cubic close-packed lattice and the cations (Mg and Al) occupying some or all of the octahedral and tetrahedral sites in the lattice.

It has an imperfect octahedral cleavage and a conchoidal fracture.

Its hardness is 8, its specific gravity is 3.5-4.1 (depending on the variety) and it is transparent to opaque with a vitreous to dull luster.

It can be virtually any color of the spectrum with various shades of red, blue, green, yellow, brown or black.

Spinel has long been found in the gemstone-bearing gravel of Sri Lanka. More recently the Badakshan Province (Afghanistan) and Mogok in Burma have supplied some of the best samples of spinels.

Other sources of gem quality spinels are also in the marbles of Luc Yen (Vietnam), Mahenge and Matombo (Tanzania), Tsavo (Kenya) and in the gravels of Tunduru (Tanzania) and Ilakaka (Madagascar).

Spinel is found as a metamorphic mineral, and also as a primary mineral in rare mafic igneous rocks.

These igneous rocks are relatively deficient in alkalis relative to aluminium so aluminium oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel and ruby are often found together.

 Some spinels are among the most famous gemstones:

• the Black Prince's Ruby and the 'Timur ruby' in the British Crown Jewels, are actually Spinels!

 Tipical shape of the crystals of spinel is the octahedron:

 Samples of faceted spinels showing part of the vast variety of colors available in nature:

 hot pink spinel from Mahenge Tanzania

 lavender violet spinel from Sri Lanka

purplish red spinel from Cambodia.

Early in the 20th century synthetic spinel was produced by the "flame fusion" process known as Vernuill method.

For many years it was used as an imitation found in birthstone jewelry and high school class rings.

Only recently, as the level of gemological awareness of consumers has grown, then spinel finally is getting some respect as the rare and beautiful gem it is.

This increased appreciation derives not only from the gem's inherent rarity and beauty, but also from the fact that virtually all spinels in today's market are unenhanced.

As more extensive and invasive treatments are given to lower grade rubies and sapphires to improve their color or clarity, spinel's natural beauty, and still relatively modest prices, become ever more inviting.

Francesco Natale
Chief Gemologist - IDL Dubai