The tourmaline is marked out from all other precious stones by a very complex chemical constitution, and by a very interesting optical structure. Its hardness, 7.3 to 7.5, suffices to protect it from wear, while the range and quality of the colours which it exhibits commend it to those persons who appreciate the artistic value of jewellery in which other stones besides those which are well known and popular form dominant elements.
All the minerals called by the names ” indicolite ” (blue), ” rubellite ” (red), ” schorl ” (black), and ” achroite ” (colourless) form but one species—tourmaline. These differences of colour are accompanied by differences of composition, so that we have a series of varieties of tourmaline, in which, while the proportion of silica is fairly constant, the bases consist of the oxides of iron, magnesium, sodium, manganese, and aluminium in differing proportions.
Water is also present, and sometimes lithia and potash. To give some notion of the chemical complexity of the tourmaline we may cite an analysis by Rammelsberg of a green Brazilian stone of specific gravity 3.107 : The specific gravity of tourmalines varies between 3 and 3.25.
The following determinations were made with particular care : The tourmaline occurs crystallised in the form of prisms belonging to the rhombohedral system ; some of the faces are striated or even channelled. The hardness of perfectly flawless transparent tourmalines is from 7 ‘ 3 up to 7 · 5.
The optical properties of tourmaline are most striking. When a crystal is viewed along the direction of its principal axis, it is less transparent and of a different colour than when viewed across that axis. The coloured varieties, or most of them, absorb and quench to different degrees the ordinary ray, which is polarised in a plane parallel to the axis, while they allow the extraordinary ray, polarized in a plane perpendicular to this line, to pass. Examples of the marked dichroism, which is so conspicuous a feature in the majority of coloured tourmalines, may be seen in this list of twin colours of the two polarized rays passing along and across the crystal respectively :
The following are some additional instances of the twin colours seen in tourmalines, owing to the optical peculiarity just named. These examples were observed with the aid of the dichroiscope, which serves for the study of such a phenomenon admirably, causing, as it does, a complete separation of the oppositely-polarized and differently-coloured rays, not attainable by mere inspection of a polished slice of a tourmaline crystal :
A few illustrations of the influence of this powerful dichroism upon the appearance of cut and faceted tourmalines will be of service, not merely in identifying doubtful specimens, but in explaining the peculiar and exquisite quality of the colours which this gem-stone shows. If we cut a green tourmaline in such a manner that the table and culet are perpendicular to the axis of the crystal, the probability is that the gem will appear, especially in its thicker parts, perfectly opaque and black.
Held sideways we may see some greenish and olive green hues, by looking across the stone from one part of the girdle to another. Now the same green tourmaline may be so cut as to present a brilliant appearance, with a fine play and interchange of two hues of green, by making the table parallel with the axis. If the crystal be a yellowish brown one, a very beautiful effect is secured by cutting it in the form of a brilliant, but with a small table parallel with the axis.
The templets and other facets of the crown should be well developed so as to display, as the stone is viewed in different positions, the different colours of the light transmitted and reflected in different directions which become visible in one after another of the facets. If one of these be at one moment greenish yellow, presently it is yellowish brown, and then russet.* With pale yellowish and greenish grey tourmalines cut in a similar manner, there will be seen other and equally striking changes of hue.
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.