_Composition_: Al₂(F, OH)₂SiO₄. _Crystal system_: orthorhombic.
_Hardness_: 8. _Specific gravity_: 3.4 to 3.6. _Luster_: vitreous.
_Color_: pale blue, sky blue, greenish, white, wine yellow, straw
yellow, grayish, pink, reddish, and orange. _Streak_: uncolored.
_Cleavage_: one direction, basal, highly perfect. _Fracture_:
conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent
to subtranslucent. _Refractive index_: about 1.60 to 1.63.
_Dispersion_: moderate.
Various yellow and smoky colored quartz gems are offered for sale as
“Spanish Topaz,” “Smoky Topaz,” “Madeira Topaz,” and “Topaz Quartz.”
These names are entirely misleading and should be dropped from usage.
Fine gem-quality white, pale-blue, and sky-blue topaz has been found
near Streeter, Grit, and Katemcy, Mason County. This Texas gem material
compares favorably in color, size, and clarity with topaz found anywhere
in the United States. Fine crystals of topaz (Pl. V, B, and fig. 18)
occasionally are found in pegmatite dikes associated with quartz, black
tourmaline, cassiterite, and pink microcline. Many of the gem-bearing
pegmatites have been eroded away, leaving the topaz concentrated in the
stream beds. The stones mostly occur as frosted, stream-worn pebbles
(Pl. VI, A) in the numerous small creeks in the area. The topaz is
heavier than the quartz and microcline that compose the stream gravel
and is commonly found immediately on top of the granite bed-rock in the
bottom of the stream bed. The stones tend to lodge behind boulders or
small dikes cutting across the stream.
[Illustration: Fig. 18. Crystal faces on topaz crystal shown in Plate
V, B. This crystal habit is typical of the topaz from Mason County.]
The white or colorless stones are by far the most common, outnumbering
the bluish stones about ten to one. The color of the blue stones tends
to be irregularly distributed in zones parallel to the crystal faces.
Topaz that is colored in this manner should be cut with the best blue
color near the bottom or culet of the gem (fig. 19). If done correctly,
this will give the entire gemstone the desirable blue color.
[Illustration: Fig. 19. Cross section showing the proper orientation of
dark-color zone in a gem cut from an irregularly colored stone.]
COLORLESS
BLUE
The colorless stones can be turned pale yellow, yellowish brown, or
straw yellow by exposure to X-ray radiation, and some of the bluish
stones will fluoresce faintly yellowish under ultra-violet light.
The largest gem-quality topaz crystal yet found in North America has
come from Mason County. It is a pale-blue crystal weighing 1,296 grams,
now in the collection of the U.S. National Museum. Several other large
pieces, some weighing over a pound, have been found. One large crystal,
exact weight unknown, was found near Katemcy. Several gem cutters have
estimated that this stone could easily yield a single, flawless
pale-blue gem of about 500 carats. Many large gems have been cut from
topaz found in this area, including at least one stone of over 300
carats.
One obstacle in the cutting of topaz is its perfect basal cleavage. The
gemstone should be oriented so that no facet of the stone will be
parallel to or within less than about 5 degrees of the cleavage
direction, or the facet may be very difficult or impossible to polish.
It is difficult to estimate the productivity of this area since its
discovery in the early 1900’s. Few systematic attempts have been made to
exploit the deposits, and a great amount of the topaz thus far recovered
has been found by private collectors. The Mason County topaz deposits
are still very productive, and additional exploration may uncover even
more gem-producing areas.
Topaz has also been found in stream gravels or pegmatites in Burnet,
Llano, Gillespie, and El Paso counties but very rarely in gem quality.