Topaz
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.
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.
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.