are known in Cornwall as
"sparable tin," in allusion to their resemblance to sparable nails,
whilst very slender crystals are termed needle-tin. Occasionally the
mineral occurs in fibrous forms, which pass under the name of
"wood-tin," and these, though not unknown in the matrix, are generally
found as rolled pebbles. By the disintegration of tin-bearing rocks and
vein-stones, the cassiterite passes into the beds of streams as rolled
fragments and grains, or even sand, and is then known as stream tin or
alluvial tin. This detrital tin-ore was probably used as a source of the
metal before the primitive miners had learnt to attack the solid
tin-bearing rocks.

[Illustration: Fig. 1]

[Illustration: Fig. 2]

[Illustration: Fig. 3]

Pure cassiterite may be colourless, or white, as seen in certain
specimens from the Malay Peninsula; but usually the mineral is brown or
even black, the colour being referred to the presence of ferric oxide or
other impurity. Occasionally the tin-stone is red. In microscopic
sections the colour is often seen to be disposed in zones, following the
contour of the crystal. A brown variety, with rather resinous lustre, is
termed "rosin tin." The usual lustre of crystals of cassiterite is
remarkably splendent, even adamantine. The mineral has a high refractive
index, and strong bi-refringence. Certain transparent yellow and brown
specimens, cut as gem-stones, exhibit considerable brilliancy. The
hardness of cassiterite is 6.5, so that it cannot be scratched with a
knife, and is nearly as hard as quartz. Its specific gravity is about 7;
and in consequence of this high density, the tin-stone is readily
separated during the process of dressing, from all the associated
minerals, except wolframite, which may, however, be removed by magnetic
separators.

Cassiterite usually occurs as veins or impregnations in granitic rocks,
and is especially associated with the quartz-mica rock called greisen.
The usual associates of the tin-stone are quartz, tourmaline, apatite,
topaz, beryl, fluorite, lithia-mica, wolframite, chalcopyrite, &c. The
presence of fluorine in many of these minerals has led to the opinion
that the tin has been derived in many cases from an acid or granitic
magma by the action of fluorine-bearing vapours, and that cassiterite
may have been formed by the interaction of tin fluoride and water
vapour. Cassiterite occurs as a pseudomorph after orthoclase felspar in
some of the altered gran