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. Tourmaline _Composition_: H₉Al₃(B·OH)₂Si₄O₁₉; hydrogen often replaced by iron, magnesium, calcium, or fluorine. _Crystal system_: hexagonal. _Hardness_: 7 to 7.5. _Specific gravity_: 2.98 to 3.20. _Luster_: vitreous to resinous. _Color_: black, brownish black, brown, blue, green, red, pink, yellow, and gray. _Streak_: uncolored. _Cleavage_: two directions, very imperfect. _Fracture_: subconchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent to opaque. _Refractive index_: about 1.62 to 1.64. Black tourmaline is schorl; brown tourmaline, dravite.