Al
Orthorhombic -- Rhombic bipyramidal
Environment
In pegmatites, seams in granitic rock, high-temperature veins and replacement impregnations, and gas cavities in rhyolite.
Crystal description
Commonly crystallized, often in free-growing transparent crystals, sometimes very large. The
c
-face, the base, may be conspicuous or may be entirely missing; it is usually present. Also in parallel columnar growths (pycnite); in pseudomorphs after feldspar crystals; and in granular masses. When doubly terminated, "precious" (golden) crystals may appear hemimorphic, for the terminal faces at the two ends of the Brazilian precious topaz are usually unlike.
Physical properties
Colorless, white, pale blue, light yellow, yellow-brown, pinkish brown, pink, and ruby red.
Luster
glassy;
hardness
8;
specific gravity
3.5-3.6;
fracture
conchoidal;
cleavage
perfect basal. Transparent to translucent.
Composition
Aluminum fluohydroxysilicate (56.5% Al
2
O
3
, 33.3% SiO
2
, and about 10% F and OH). The paragenetical habit differences are great enough to suggest major structural differences, perhaps related to the F and OH content.
Tests
Infusible, and insoluble in all but hydrofluoric acid. The powder turns blue (aluminum test) when moistened with cobalt nitrate and heated.
Distinguishing characteristics
Great hardness and its good cleavage are excellent indications, along with its crystal form and typical occurrence and pegmatitic associations. Beryl fuses on thin edges; quartz decrepitates more violently and easily.
Occurrence
A valuable jewelry stone, especially in the pink, red, and brown hues. Not to be confused with the brown quartz commonly sold in Brazil and other lands (under the name topaz, as in "smoky topaz."
Large topaz crystals are not common in the U.S.; the biggest are probably some crudely shaped white ones found in the pegmatite at Amelia, Virginia, associated with microcline. Clear crystals, to 4-5 in. (10 cm) across and somewhat etched, from Devils Head and Pikes Peak, Colorado; smaller crystals have come from other Colorado localities. Large and deeply etched blue crystals were found at Topsham, Maine, and many 1-2 in. (2-5 cm) crystals were found in small miarolitic cavities in granite with smoky quartz, feldspar, and phenakite at Baldface Mountain, New Hampshire. Less etched, pale blue crystals are found in Mason Co., Texas. Also important in San Diego Co., California, dikes associated with beryl and tourmaline.
Rhyolite extrusions of the Thomas Range, Utah (and, less conspicuously, of Nathrop, Colorado, and New Mexico) contain many gas cavities in which there are 1-in. (3 cm) light brown crystals, which soon fade to colorless on exposure to light. Larger but sandy crystals within the rhyolite, mostly filled with quartz and opaque, are always simpler in their terminations. It is associated in Utah with black bixbyite ([Mn,F]
2
O
3
) cubes, rosy beryl (not morganite), and in Colorado with spessartine garnet. In San Luis Potosí, Mexico, similar and sometimes larger topaz appears to occur alone or perhaps with cassiterite in the rhyolite seams.
Brazilian topaz is outstanding and comes in pegmatitic colorless and blue crystals, as well as in a series of quartz veins in rich brown gemmy crystals known as "precious topaz." Pink crystals of this type have also been found in nearby hematite mines and lately in Pakistan. Small natural pink Russian topazes from the valley of the Sanarka River resemble some of the brown Brazilian gemstones, with the same crystal habit. Fine blue topaz crystals (and sherry-hued fading to blue) were found years ago in pegmatites in the Ural Mountains; lately large numbers of pale brown crystals, fading to white, have been found in Pakistan.
Though like pegmatite crystals in habit, the small crystals cut for the Saxon crown jewels were straw-hued topazes from veins in and near the "Schneckenstein" (snail stone, from its shape) in the Erzgebirge near the Czechoslovakian border.
Remarks
Golden Brazilian "precious" topaz turns pink on heating, and most pink jewelry topaz has been heated, though recent finds in Pakistan are naturally pink. Topaz is a very attractive and abundant species, characterized by beautiful crystals, placing it among the more popular gemstones with collectors. The common deep blue topaz now so abundant in the jewelry trade is largely irradiated white material; no naturally blue topaz ever reaches such a rich hue.