Topaz, Visiting The Mine

During the eighteenth century more than half of the world's supply of gold was removed from the verdant hillsides surrounding the quaint cobble-stoned Brazilian town of Oro Preto. These same hills hold almost the entire world's known commercial reserves of Imperial and Precious topaz.

Photo: R. W. Wise

Oro Preto – formerly Villa Rica (Rich Town), a city that had a larger population and per capita income than New York in the 1800’s.

Small deposits of low-grade topaz have been found in the northern Brazilian state of Para, in Mexico, Ceylon, Burma, and Pakistan and in Russia's Ural Mountains. Oro Preto, the name translates as "Black Gold", is the only location that is currently producing commercial quantities of natural topaz.

Before going further some clarification is in order. This discussion is about natural color topaz as distinguished from blue topaz. Natural blue, in fact any topaz in the blue/green color range is an extreme rarity in nature. The blue topaz, which seems to be everywhere in the market is common colorless topaz that has been color enhanced through radiation and heat treatment.

Natural color topaz is usually divided into two types; “precious” topaz and “imperial” topaz. There is some confusion as to the distinction: Some experts consider precious topaz to be any topaz in the yellow color range. Others limit the term precious topaz to stones that do not exhibit the strong multicolor effect that is normal in the imperial variety.

Topaz is strongly dichroic. The C axis of the crystal is usually darker than the AB axis. So, when cut with the AB axis face up, particularly in the long pear, oval and marquee shapes that insure the best yield from the rough, the darker hue of C axis bleeds into each end of the gemstone showing a richer more saturated color at each end of the finished gem.

Topaz is a fluo-silicate of aluminum that crystallizes in the orthorhombic crystal system. Topaz has a refractive index of 1.6-1.7, specific gravity of 3.5-3.6 and a moh's hardness of 8. Topaz has perfect cleavage, that is, it parts easily parallel to its cleavage planes and is therefore despite its hardness rated poor in overall toughness.

Hue:

Natural color topaz occurs on a color/rarity continuum from yellow through to orange, cinnamon-pink (peach), orange-pink (ripe-peach), pink, dark orangy red (hyacinth) and violetish red.

Saturation:

Brown is often encountered as a secondary hue in topaz. The brownish-yellow to yellow-brown is sometimes called Sherry topaz. Prices follow this same line with yellow priced lowest and violetish red fetching the highest prices.
The gem can occur as a mixture of any of the above hues. So, the possible colors of topaz are theoretically infinite. The peach and cinnamon colors are the most characteristic. Topaz possesses a soft velvety liquid brilliance that is unique to this gem species.

Photo: courtesy ICA.

“Imperial” topaz with the characteristic ripe peach hue. Note the darker toned richer hue toward then ends of the gemstone.

Tone:

Using a tonal scale where window glass in 0% and coal is 100% tone, the optimum tonal range for topaz is 60-75%. Below 60% the color begins to pale and wash out; above 75% topaz begins to loose the liquid affect and appear overdark.
The only known color enhancement for topaz is heat treatment. This technique known as "pinking" is performed under relatively low temperatures, at times over the open flame of a miner's campfire. Stones with some pink or bluish pink can be turned a purer hue using this technique. Although experienced dealers claim to be able to separate heated from non-heated stones by eye, pink topaz also occurs naturally and at the time of this writing there is no gemological test that can identify "pinked" topaz.

Topaz is a night stone. That is, it looks its best in incandescent or candlelight and it holds up well in low-light environments. Topaz tends to bleed a bit, that is loose both color and tone in florescent light and in daylight. Quartz-halogen lighting, which due to its natural looking white spectrum is replacing the old-fashioned incandescent spot in most jewelry stores, is a bit kinder to topaz.

A majority of the topaz currently on the market can be traced to a single mine, Capão, located about five kilometers from the small village of Rodrigo Silva almost dead center of the two hundred ninety square kilometer topaz belt running in an east-west direction west of Oro Preto.

Topaz mines around Oro Preto, the original geological setting.

Capão, the name means "big lid" in Portuguese, is one of two large-scale mechanized mines currently operating. It is located just outside Oro Preto near the village of Rodrigo Silva. A second large mechanized mine, Vermelhão, is situated about 12km to the east.

Capão is an open pit operation with two large pits, each carved over two hundred feet down into the verdant hillside. The mine currently employs forty-seven workers and is highly mechanized. Bulldozers are used to open the pits and most initial sorting is carried on using German made hydraulic sluices and sieves with some final sorting done by hand. The huge amount of water necessary for mining operations is drawn from a nearby lake. Mining technology is basic. The chief difference between modern hydraulic mining and old fashioned panning lies not in the methodology but in the ability of machinery to process large amounts of gem bearing gravel quickly.

Photo: R. W. Wise

Gold panning is still practiced in the steep valleys outside Oro Preto. This method is also used to pan for topaz. Topaz mine outside the village of Rodrigo Silva, Minas Gerais, Brazil.

There are a number of smaller mines in the area including Don Bosco and Garimpo that are strictly hand operations that produce intermittently. The only other large-scale operation is the Vermelhão mine located just twelve miles away.

 

 

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