Thursday, 20 June 2013

Grossular Garnet (Ca3Al2Si3O12)

Andradite garnet
Silicate of calcium and aluminium, belonging to the garnet group. The name grossular is due to the fact that some of these crystals resemble gooseberries, the Latin name for which is grossularia.

Crystal system

Cubic

Appearance

Grossular also has the typical crystal form of garnets, occurring in isolated crystals which are often complete, in the shape of a rhombic dodecahedron, sometimes combined with a trapezohedron. They vary in transparent to serniopaque. The typical color is light (gooseberry) yellowish green; but they can be a strong to bluish green, honey yellow or pinkish yellow, or even perfectly colorless. When transparent, the crystals have god luster. Like other garnets, they have no cleavage. The greenish to yellowish varieties are used as gems.

Physical properties

HessoniteThe hardness is 6.5-7.5 or a little more. The density is somewhat variable: from 3.58 to 3.69 g/cm3. The refractive index is about 1.74. Occurrence Grossular is not a rare mineral. The type; used as gems mainly come from the gem gravels of Sr. Lanka (honey yellow variety); and the United States. Canada, Mexico, Madagascar, Kenya, and Tanzania (green variety).

Hessonite 

The yellow-brown variety of grossular is called hessonite (or essonite). Its name comes from the Greek meaning "inferior," gems of this color being regarded the least valuable.

Appearance

Tsavolite
It is a honey-yellow or yellow-brown sometimes tending to a pinkish orange similar to tiding spessartine. It has good luster and seemingly good parency, but when viewed with a lens or other magnification, the interior always looks "treacly," dulating, contorted areas of lesser transparency, highly concentrated sugar solution with frequent, transparent crystalline inclusions. The gems are norm given a mixed, oval, or round cut.

Distinctive features
Seen through a lens, the treacly appearances combined with the color are sure memo identification. Nothing comparable is found in other of similar color, such as citrine quartz, topaz, and sapphire. Its luster, in any case, is superior to that of quartz. It is distinguished from zircon of a similar cold its lack of obvious birefringence. Occurrence It mainly comes from Sri Lanka, but it found in the United States, Canada, and Brazil.

Value

The value of hessonite is rather low, like that mandine and pyrope, despite its very attractive ance.

Simulants and synthetics

It has neither been nor produced synthetically.

Green Grossular

The green variety of grossular garnet, discovered a le decades ago and found mainly in Kenya, near the Tsavo National Park, is also known as Tsavorite (or Tsavolde)

Appearance

It is a light, verdant, or dark green, shit to the color of the better green tourmalines and sometinn it is said, even comparable to African emeraid. It hasps luster. These gems, which are usually given a round o. pear-shaped mixed cut, or occasionally a brilliant cut generally small, rarely exceeding one carat and new more than a few carats.

Distinctive features

Green grossular
Being singly refractive, green grossular is distinguished from green tourmaline, by the letters strong birefringence and pleochroism, and from melt green zircons, which are obviously birefringent, whets measurement of the physical properties is necessary  distinguish it from green sapphire when the latter doesrd display clear pleochroism. It is very similar in all respectst a recent artificial product of comparable structure, namely green YAG (Yttrium Aluminium Garnet), from which it distinguished by its physical properties.

Occurrence

It is very rare; being found mainly in Kenya and Tanzania, but also in Pakistan.

Value

If a good color (a lively, strong green), it can to the top price bracket for secondary gems; this is especially true of the very rare examples weighing a few carals. Little known by the general public, it is in demand by collector, and connoisseurs.

Simulants and synthetics

Green grossular has only been known for a few decades. Green YAG fan (an artificial product with the structure of garnet, but not containing'''. silicon) closely resembles it and can be a good imitation. It is not produced synthetically.

Writer – Curzio Cipriani and Alessandro Borelli

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