Ch. 5: Color, Luster, Beauty

Ch. 4: Forms of Crystals Page of 187 Ch. 5: Color, Luster, Beauty Text size:minus plus Restore normal size   Mail page  Print this page
distinguished 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.
Compound crystals.—Crystals 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.
Chapter V
GEM-STONES 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 these particles.
At 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
Ch. 4: Forms of Crystals Page of 187 Ch. 5: Color, Luster, Beauty
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