Discrimination of Gems
By T. S. G. Kirkpatrick
Author of 'The Hydraulic Gold Miner's Manual'
E. & F. N. Spon, 125 Strand
Spon & Chamberlain, 12 Courtlandt Street
As a general rule each miner follows a special "quest," and, either from ignorance or absorption in his one pursuit, passes unnoticed valuable stones which would add considerably to the success of his labours.
Fifty years ago the South African hunter trampled on diamonds, and scrambled over gold reefs in his anxiety to obtain a skin, a pair of horns, or an ostrich feather.
Two noted experts, the one English the other American, sent expressly to report, stated their conviction that no indications existed at Kimberley of any large output of diamonds.
But in the last decade upwards of thirteen tons of diamonds have been exported from the Cape, as against one ton during the last 100 years from Brazil and elsewhere.
The big game has vanished or retreated up-country, while millions sterling have been added to the world's wealth from these unnoted sources.
The night is made hideous by the clank of stamps instead of the roar of the lion, and the ladies of the east-end of London flaunt it in the feathers of the "farmed" ostrich, and "sport" real diamonds from Kimberley (somewhat "off colour," no doubt, but still real diamonds) instead of cocks' feathers and glass!
The compiler of these following notes thought that they might serve to accentuate the moral of the tale of
EYES AND NO-EYES.
London: February 15, 1895.
Discrimination of Gems.
THE following is intended as a ready method of discriminating between different gems having the same outward appearance, but differing essentially in many particulars which constitute their market value.
As, in nature, they are rarely found in a condition "to classify by inspection of their crystallisation," other proofs must be sought before deciding their value.
In many instances the same crystalline form, or the same colour, is found in totally different stones, while in others differing colours render it difficult to recognise the identity of essentially similar stones. We must therefore get the assistance of other factors before pronouncing judgment.
Of these, the most important sources of recognition are specific gravity and hardness.
Specific gravity means the density or proportion of weight to volume of a substance, and is a characteristic of all substances; while the varying hardness is discovered by a simple rule of thumb, easily obtained by practice.
Presuming that the reader is acquainted with the crystalline forms, such as the cube, the octohedron, &c., and their various modifications—the cube having six faces square; the octohedron eight sides, equal-sided triangles; the dodecahedron twelve-sided diamond-shaped rhomboids; the tetrahedron four sides, each triangular; the rhombohedron six sides, each rhombic; the prism, any column with three or more sides, which when placed on its base, may stand straight or oblique, terminating abruptly with a flat face, or coming off to a point, blunt or sharp, like a pyramid—he will be able, if fortunate in finding perfect specimens, to recognise the substance by simple inspection, combined with colour.
Many minerals break, when sharply struck with a hammer, in certain definite directions—the planes of their crystallisation—or "cleavage"—but if irregularly, they exhibit what is called "fracture," usually conchoidal.
Some minerals have a brilliant lustre like that of metals, while others are glassy, silky, waxy, or resiny; others are dull and quite destitute of lustre. The lustre of a diamond is called adamantine.
Some minerals are colourless, white, black, or any colour, either dull or brilliant, and the same mineral may have a variety of tints or colours. Some in mass having distinct colour, when scratched, give a powder altogether different from the mass: in other words, the "streak," as it is called, may correspond with, or differ altogether from, the mineral operated on—e.g. the sapphire has a blue colour, but a colourless streak. In some cases the streak is metallic. Tinstone (oxide of tin) gives a whitish-grey streak, which at once distinguishes it from minerals having the same outward appearance; and an acquaintance with this fact would have saved many prospectors from disappointment, as hundreds of samples absolutely free from tin have been sent to England under the impression that they were that valuable mineral instead of tungstate of iron, &c. The colour and transparency of a gem are best seen by immersing it in water, half-an-inch below the surface. Specific gravity coupled with hardness is the most reliable test.
The rule for finding the specific gravity is to deduct the "weight in water" from the "weight in air," and divide the "weight in air" by the difference, e.g.:—
|Quartz weighing||1398||grains in air|
|862||grains in water|
For hardness a scale has been adopted, of which the following is a whimsical "memoria technica," and which, though not accurate in the consonant denoting the mineral, still is sufficiently suggestive to recall the desired symbol:—
The first four may be easily arrived at by testing with a steel knife; the next two with difficulty; but 7, 8, 9, 10 refuse to yield to steel, and it is therefore necessary to be provided with specimens of quartz, topaz, corundum (sapphire), and diamond, with sharp edges, to make the test.
The diamond is the hardest substance known, while the topaz is cut by the sapphire, and quartz by the topaz.
No valuable gem is softer than quartz, or has a lower specific gravity, except opal, which is 2·2.
No mineral which scratches quartz has the curved edges of the diamond. Therefore any mineral which scratches quartz, and has curved edges, is a diamond.
A great number of stones identical in composition occur of various colours—white, yellow, brown, black, red, violet, green, &c. In the following table of specific gravity and hardness the different stones are arranged under the different colours, and the characteristics of those which occur under more than one head are not repeated, but referred back.*
*In some instances these figures represent the average, and stones may vary two or three decimals.
|Brown and Flame-coloured Stones|
|6·5 to 7·5
6·5 to 7·5
|Red and Rose-coloured Stones|
|Disthene (cyanite)||3·3||5 to 7|
|Dichorite (water-sapphire)||2·6||7 to 7·5|
|Turquoise||2·8 to 3||5 to 6|
|Chrysolite||3·3||6·5 to 7|
|Chrysoprase (see Quartz)|
|are easily recognized by their play of colour.|
|Tourmaline: this stone, when broken, looks a little like black resin.|
The diamond, sapphire, garnet, tin ore, gold and platinum are common in river sands.
Platinum is easily recognised by its great weight (the specific gravity being fully as high as gold) and by its steel-grey colour. It is usually in flattened pieces, rather larger than a pin's head.
Tin ore (oxide of tin) has a peculiar resiny look, a specific gravity of 6·8 to 7, and hardness 6 to 7. If powdered, and mixed with twice its weight of cyanide of potassium and one-fourth its weight of charcoal in a crucible, and 50 or 60 grains of cyanide of potassium sprinkled on the top, the heat of a kitchen fire is sufficient to produce a globule of tin.
All polished gems become more or less electric by rubbing on cloth, but the diamond in its natural state and the topaz become very highly electric; and the topaz retains its electricity for so long a time, that it is a very useful test by which to recognise it. It is also pyro-electric—that is, becomes electric on the application of heat by the blow-pipe, or even by a candle. This distinguishes it from the diamond, which has nearly the same specific gravity.
Tourmaline, too, becomes polarly electric by heat, attracting and repelling on the opposite sides.
Zircon, which is used in watchmaking as "rough diamond," is nonelectric, as is also the sapphire. So that these five principal stones can be easily distinguished.
The diamond gives a peculiar sound when two are tapped together, which is easily recognised by those who are in the habit of handling them.
Quartz gives a peculiar smell when its pebbles are rubbed together, called by school-boys "firestone smell".
All the above characteristics will, if carefully noted, enable a novice to distinguish between the different gems, if found in a rough state or imperfectly crystallised form.