by indices, which are given by the relative lengths the faces intercept
on the axes of reference or one or two faces may be parallel to a face.
may be single or individual or may be of several individuals, i.e.
compound crystals: (i) parallel groups, (ii) twin crystals. In (i) the
crystal axes are parallel. In (ii) the crystals have intergrown in a
special manner. A twin may also consist- of more than two
individuals—they may be of the contact type or the penetration type.
COLOUR, LUSTRE, BEAUTY, ETC.
are made very attractive by their colours, and some have to depend
solely on their colour for their charm and appeal. Colour is also
influenced by the light used and on the degree of colour vision of the
observer. Colours are seen well in white light. The colour of
gem-stones in most cases is accidental and not essential. Most
gem-stones in a pure state are without colour. The colouring matter
may consist of various metallic oxides in quite small amounts.
Wonderful colour effects have been produced in synthetic stones by the
addition of metallic oxides. There is also a theory that coloured
crystals contain colloidal particles, either of foreign matter or of
the crystal itself and that the colour seen depends on the size of
times the colour may be unevenly distributed and may occur as irregular
patches or blotches as is often the case with amethyst and sapphire.
The colour may also be distributed in regular bands or zones. The
variety of quartz known as agate shows'banding with different colours.
Inclusions may also produce colours, as in moss agate.
In some gem-stones the colours disappear on heating and could be restored by exposure to radium. Some gem-stones