spoken
of as pleochroic. It is transparent to translucent, and has a vitreous
lustre on the crystal faces. It is doubly refracting, the greatest
index for yellow light being 1'543, and the least 1.537. The dispersion
is feeble. It is only fusible at high temperatures, and it is but
slightly attacked by acids. The specific gravity varies from 2-60 to 2-72,
depending largely on the amount of iron present. It has a conchoidal
fracture, and a hardness of 7 to 7-1/2. It has a distinct cleavage
normal to the longer horizontal axis, and indistinct cleavages parallel
to the other two axes. In crystalline form it is rhombic, but distinct
crystals of any size are rare, and when they are found are usually
rough. The general habit of the crystal is prismatic, and rather short.
Twinned crystals sometimes give rise to a pseudo-hexagonal form {see Chrysoberyl).
More often the mineral occurs in rolled fragments or massive.
Occasionally comÂpletely developed crystals are found in granite, as in
Bavaria, but usually it is only found in association with rocks which
have undergone great metamorphism.
In
chemical composition it is a silicate of aluminium, with magnesium and
iron, and often with calcium, and containing water. The formula is
possibly H2O4 (Mg, FeO), 4 A1203, 10 Si02.
The colour is probably due to ferrous iron. The most commonly
associated minerals are Quartz, Orthoclase or Albite, Tourmaline,
Garnet, Andalusite, Beryl, Topaz, etc. It occurs in many parts of
Scandinavia, as at Arendal and Kragero in Norway; at Abo in Finland ;
at Tunaberg in Sweden; in Greenland, etc. But the chief gem qualities
come from the gem gravels of Ceylon ; there stones of both light and
dark blue are found, specimens of a sky blue being known as "Water
Sapphire or Saphir d'Eau,
