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are
striated parallel to the-vertical axis of the crystal. Usually one end
of the crystal is terminated, the other being a cleavage plane and the
crystals grow on one end attached to the matrix. Other faces present
are the pyramid and the domes. The appearance of the crystals becomes
different due to variation in relative sizes of these faces, but the
interfacial angles and directions of edges are constant. also
occurs in compact and granular masses and water-worn crystals. The
basal cleavage is parallel to the basal pinacoid and is an important
character. It is also seen by well-defined cracks. This has to be
considered when cutting . The fracture is conchoidal to uneven.
is clear and colourless, but may at times be coloured. Colours
may vary from wine yellow, brownish, grey and pale tints of blue to
green, violet and red. Pale blue is common, but green, red and
pink are rare. Crystals of a sherry yellow colour are known only from a
single locality in Brazil—Brazilian — most of the Brazilian
is colourless t»r blue. The yellow is of gem variety. Lustre:
Vitreous. Specific gravity =
a =
0-010. Dispersion is
position : It is a fluosilicate of aluminium
On account of its hardness, it takes a good polish and shows a brilliant lustre which is often seen on the crystal faces.
Grades
(1) The Brazilian has a sherry yellow colour and is of gem quality, but much of it is also of pale blue colour.
(2) Crystals of a brownish yellow shade occur in Siberia, Utah and Colorado, but they fade on exposure to light.
(3) Ceylon is colourless but is confused with yellow sapphire as 'oriental '.
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