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
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.
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
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.
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
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