OPAZ crystallizes in
the orthorhombic system, and occurs in prisms with one end regularly
terminated, and has a very perfect cleavage transverse to the prism.
Its hardness is 8, and specific gravity 3'53. It is a silicate of
alumina containing fluorine. A blue crystal weighing 20 pounds is in
the Imperial Mining School at St. Petersburg, Russia. Fine blue and
sherry colored crystals have been found in Siberia, blue ones in
Scotland and Ireland, yellow in Minas Geraes, Brazil ; white in Villa
Rica, Brazil ; and blue and white in Ceylon and Australia. Brazilian or
true mineralogical topaz is often confounded with two other minerals,
namely, citrine and Spanish or Saxon topaz, the color of which is made
by heating and so decolorizing smoky quartz to various shades of yellow
or brown. Yellow sapphire is called Oriental topaz. The specific
gravities of the three varieties are given for comparison.
True yellow topaz, if heated for a time, becomes pink, and continued heating renders it colorless.