TOPAZ Al 2 SiO4 (F,OH)2
Crystal Description: Orthorhombic
Crystals are elongated prisms, most of which are terminated;
may form clusters or even occasionally doubly-terminated crystals.
Three types occur within the Thomas Range: transparent, rough opaque
and smooth opaque. Most crystals are 0.5 inches or less in length; larger
crystals are uncommon. Often in association with or in combination
with other minerals.
Color- Thomas Range topaz varies from water clear to a rich sherry
amber to light pink; some crystals are dichroic, showing a dark amber
color when viewed perpendicular to the C-axis and a deep cinnamon
red color when viewed directly down the C-axis.
Specific Gravity- 3.5
Cleavage- Perfect basal
Unfortunately, the color of the Thomas Range topaz is not stable
when crystals are left exposed to direct sunlight for extended periods
of time. After being exposed to direct sunlight, the crystals change from
the rich sherry color to a light pink and eventually to a clear silver-
white color; it takes about a week to ten days for the change to take
place. This is why all of the countless millions of crystals so plentiful
on the hillsides and in the washes are of a clear, silver-white color.
The crystals can, however, regain the original sherry color upon
exposure to strong radiation for a short period of time. The sherry color
of the unexposed crystals is a direct result of exposure to naturally
occurring ground radiation for millions of years, probably from trace
amounts of uranium in the rhyolite. Radiation causes electrons to be
displaced to a higher energy state giving the crystal a temporary color
center. Exposure to direct sunlight excites the electrons causing them
to return to their normal state, thereby eliminating the color center,
resulting in a color shift from sherry to colorless.
Most people are under the misconception that once the topaz
crystals are exposed to daylight the color is automatically doomed. It
has been my experience, over the past 25 years, that when proper care
is taken, and direct sunlight is avoided, the color will remain almost
indefinitely. Artificial lighting, including fluorescent and halogen light,
does not appear to have any negative effect on the color stability of the