Minerals from Pakistan
....Topaz.....

Topaz is an extremely hard fluorosilicate of aluminum having formula Al2(F,OH)2SiO4.

It is a birthstone for November.  In the past topazes have been used to cure bad vision and to ward off death. Topazes were also used in the middle age against the Bubonic plague. In the ancient medicine the powder of this mineral, mixed with wine was used to prevent asthma and insomnia.

Topaz is still the stone of the intelligence, long life, beauty and loyalty: a topaz shouldn't miss in your collection !!!!!.
 

Topaz is probably the most common among the gems, it forms by fluorine-bearing vapors created during the crystallization of igneous rocks, so its typical occurrence is in  volcanic rocks ( rhyolites)  and in pegmatites.
 

   The mineral is very common and the name topaz is still now sometimes used to indicate the citrine or smoky quartz. In general the name is used in low level jeweler to indicate all the yellow stones. Sometimes the mineral is confused with the quartz, but it is well differentiated especially by the crystallization and also by the cleavage, perfect along the z axis in the topaz.

The name is derived from an island in the Red Sea, very famous in ancient times for the peridot production, but without any interesting occurrence of topaz.

The crystallisation is in the Orthorhombic system, with crystals with well-formed prisms, with a basal pinacoid on one side and a small pinacoidal face surrounded by pyramid and horizontal prism faces at the other. The hardness in the Mohs scale  is 8, so the topaz is one of the hardest minerals and also this is a characteristic that differentiate this mineral from quartz ( hardness 7 ) . Despite of the hardness, the toughness is fair, sometimes

the mineral is due to cleavage and is not uncommon to broke the crystals during cleaning.The specific gravity is from 3.5 to 3.6 . The mineral is weakly fluorescent to the UV
 

COLORS

The rarest topaz are the reds, intense pinks or violets, that are also,clearly the most expensive, followed by the imperial topaz. . In terms of value, then there are the yellow, more or less intense, to brown and finally the msot common topaz are the blue ones. The blue topaz has substituted the aqua as the cheaper blue gem , in fact one of the characteristics of this mineral is the fact that it is very often treated to change the color of the stone. The most common techniques to change the color are  irradiation and heating. The biggest part of the natural topaz are colorless to yellowish to very pale blue. The dark blue gems are normally obtained by irradiation and heating. The most common colors are yellow and blue, but other colors might be transparent, and brown to smoky. The most researched colors are pink to deep  pink to red. The only varietal name is the  Imperial Topaz, of a strong orange to red color, very nice and very rare in nature .

 A lot of different techniques are used to enhance color in topaz. These treatments substantially reproduce in laboratory the natural ways to give the color to this stone, so there isn't any scientific and definitive method for differentiating among natural and trested stones .

The most common techniques are:  
Nuclear irradiation - the mineral is treated with fast neutrons. The color obtained is normally deep blue. Pay attention to the fact that the topaz treted in this way is normally radiactive for several months after the treatment and should be stored before of selling it .
Gamma rays cells - the mineral is exposed to gamma ray sources to get, after heating, light blue colors . Before heating the color is dark yellow-brownish, but it is unstable .  Linear Accelerators , where the treatment consist with exposure to a high energy electron beam. The final color is blue, darker than the gamma rays and lighter then the nuclear irradiated one ( commonly called sky-blue ) . Process generates radioactivity that anyway decades after few weeks .

The orange and brown colors normally tend to fade with strong exposure to the direcr sunlight.

Sources

The mineral is typically associated with the late-stage of crystallization of igneous rocks, with high fluorine concentrations, typically some rhyolites and the pegmatites. In pegmatites it is often associated with muscovite, tourmaline, albite, quartz,  fluorite, and beryl.
Typical, hystorical  occurrence is Brazil (Minas Gerias) where also huge crystals have been found and where there is the rare Imperial Topaz. In practice all the Brazilian blue topaz have been treated . has be treated (see below) to enhance or alter the original color. In central America Topaz are found in Mexico, near San Luis Potosi
 in rhyolitic volcanic rocks ( deep yellow, brown to pale pink ) . In Asia we have as usual Sri Lanka, from the gem placers, where topaz is found togheter with ruby, sapphire and beryl. Other typical occurrence of the last years is Pakistan, where in Pegmatite we have probably the best samples on matrix of this gem. The color vary from colorless to more or less intense yellow, to brown to pink . Other classical source for crystals on matrix are the U.S., from the pegmatites of Pikes Peak, CO and Streeter , TX, and from the classical occurrence in rhyolites of Thomas Range, in Utah.

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