In the table on page 76 several localities of tourmaline are given. Of these some yield crystals, which are parti-coloured, perhaps having a rosy central position enclosed in a green shell. Recently many fine tourmalines have reached this country from Heemskirk Island, Tasmania. Some of the green crystals from this locality are large and clear : tourmalines of peacock-blue and apricot-yellow have also been found. The Heemskirk green crystals are well suited for polariscope purposes.
When a tourmaline is rubbed, or, better still, when it is heated, it becomes electrically charged. The polarity of the charge is beautifully shown when a mixture of red lead and sulphur in powder is allowed to fall from a muslin sieve upon a tourmaline crystal when in process of cooling. The red lead will gather about the negatively electrified end, the sulphur about that which is positively electrified.
The great group of the garnets includes several gem-stones which would not be included under a single name, as having many characters in common, were it not that chemical and crystallographic properties must be allowed to overbalance the more obvious peculiarities of these minerals. Garnets present almost all hues and tones of colour save those in which blue predominates, while they vary greatly in hardness and specific gravity. But the crystalline forms in which they occur are all referable to this same system, the cubic or monometric, while the chemical expression which represents their constitution is identical in structure, though one or another constituent be replaced by analogous elements. All garnets are normally singly refractive and monochroic ; where double refraction is observed it is due to internal stresses. The following list includes the chief varieties of garnet :—
1. Cinnamon-stone or Hessonite—Calcium aluminium garnet.
2. Almandine and Carbuncle—Ferrous aluminium garnet.