Another property by which the various gemstones may be distinguished from each other is hardness
. Hardness is the ability to resist scratching. The term "hardness" should not be taken to include toughness as is commonly understood by the public. Most hard stones are more or less brittle and would shatter if struck a sharp blow. Other hard stones have a pronounced cleavage
and split easily in certain directions. True hardness, then, implies merely the ability to resist abrasion i. e. scratching.
Not only is hardness very necessary in a precious stone in order that it may receive and keep a fine polish, but the degree in which it possesses hardness as compared with other materials of known hardness may be made use of in identifying it.
No scale of absolute hardness has ever come into general use, but the mineralogist Mohs many years ago proposed the following relative scale (see Table 1 for comparison between Mohs relative scale of hardness and absolute hardness), which is still in common use.
Diamond, the hardest of all gems, was rated as 10 by Mohs. This rating was purely arbitrary. Mohs might have called it 100 or 1 with equal reason. It was merely in order to represent the different degrees of hardness by numbers, that he picked out the number 10 to assign to diamonds; Corundum Mohs called 9, as being next to diamond in hardness; Topaz he called 8; Quartz was given the number 7; Orthoclase was rated 6; Apatite 5; Fluorite 4; Calcite 3; Gypsum 2; and Talc 1.
Table1: Hardness of the Principal Gemstones
Mohs's Scale of Hardness.
Any mineral in this series, that is of higher number than any other, will scratch the other. Thus diamond (10) will scratch all the others, corundum (9) will scratch any but diamond, topaz (8) will scratch any but diamond and corundum, and so on.
It must not be thought that there is any regularity in the degrees of hardness as expressed by these numbers. The intervals in hardness are by no means equal to the differences in number. Thus the interval between diamond and corundum, although only one number of difference, is greater than that between sapphire (9) and talc (1) - see Table 1. The numbers in Mohs scale merely give us an order of hardness. Many gem minerals are, of course, missing from this list, and most of the minerals from 5 down to 1 are not gem minerals at all. Few gem materials are of less hardness than 7, for any mineral less hard than quartz (7) will inevitably be worn and dulled in time by the ordinary road dust, which contains much powdered quartz.
In testing a gem for hardness the problem consists in finding out which of the above minerals is most nearly equal in hardness to the unknown stone. Any gem that was approximately equal in hardness to a topaz (8) would also be said to be of hardness 8. Thus spinel is of about the same hardness as topaz and hence is usually rated as 8 in hardness. Similarly opal, moonstone, and turquoise are of about the same hardness as feldspar and are all rated 6.
Frequently stones will be found that in hardness are between some two of Mohs's minerals. In that case we add one half to the number of the softer mineral; thus, peridot, benitoite, and jade (nephrite) are all softer than quartz (7) but harder than feldspar (6); hence we say they are 6.5 in hardness. Beryl (aquamarine and emerald), garnet (almandine), and zircon are rated 7.5 in hardness, being softer than topaz but harder than quartz.
How to Apply the Hardness Test. The beginner should take care against damaging a fine gem by attempting to test its hardness in any but the most careful manner. The time-honored file test is really a hardness test and serves nicely to distinguish genuine gems, of hardness 7 or above, from glass imitations. Glass imitations are easily attacked by a file; a well-hardened steel file is of not quite hardness 7, and glass of various types, while varying somewhat, averages between 5 and 6. To make the file test use only a very fine file and apply it with a light but firm pressure lengthwise along the girdle (edge) of the unset stone. If damage results it will then be almost unnoticeable. Learn to know the feel of the file as it takes hold of a substance softer than itself. Also learn the sound. If applied to a hard stone a file will slip on it, as a skate slips on ice. It will not take hold as upon a softer substance.
If the stone is set, press a sharp corner of a broken-ended file gently against a back facet, preferably high up toward the girdle, where any damage will not be visible from the front, and move the file very slightly along the surface, noting by the feel whether or not it takes hold and also looking with a lens to see if a scratch has been made. Do not mistake a line of steel, left on a slightly rough surface, for a true scratch. Frequently on an unpolished girdle of real gem material the file will leave a streak of steel. Similarly when using test minerals in accordance with what follows do not mistake a streak of powder from the yielding test material, for a true scratch in the material being tested. The safe way is to wipe the spot, which will removing any powder. A true scratch will, of course, persist.
A doublet, being usually constructed of a garnet top and a glass back, may resist a file at the girdle if the garnet top covers the stone to the girdle, as is sometimes the case, especially in the smaller sizes. In this case the back must be tested.
A file should never be passd rudely across the corners or edges of the facets on any stone that may be genuine, as such treatment really amounts to a series of light hammer blows, and the brittleness of most gem stones would cause them to yield, irrespective of their hardness. It should be remembered that some genuine stones are softer than a file, so that it will not do to reject any material that is attacked by a file as worthless . Lapis lazuli (5), sphene (5), opal (6), moonstone (6), amazonite (6), turquoise (6), peridot (6.5), demantoid garnet (6.5), and jade (nephrite) (6.5), are all more or less attacked by a file
Minerals Used in Testing Hardness. The following set of materials are used for testing stones that are harder than a file:
- A small crystal of carborundum.
- A small crystal of sapphire.
- A small topaz crystal.
- A small quartz crystal.
- A fragment of a crystal of feldspar.
These five test stones represent the following degrees of hardness:
- Carborundum is harder than any gem material but diamond. It will scratch sapphire and ruby, which are rated 9 in hardness, hence we may call carborundum 9.5 if we wish. It is, however, very much softer than diamond, and the latter will scratch it upon the slightest pressure.
- Sapphire, of hardness 9, scratching any gem material except diamond.
- Topaz, of hardness 8. It is scratched by sapphire (and, of course, ruby), also by chrysoberyl (which is hence rated 8.5), but scratches most other stones. Spinel (which is also rated as 8 in hardness) is really a bit harder than topaz.
- Quartz, of hardness 7, and scratched by all the previous stones but scratching those that were listed above as of less hardness than a file.
- Feldspar, of hardness 6, hence slightly softer than a file and yielding to it, but scratching the stones likewise rated as 6 when applied forcibly to them. Also scratching stones rated as less than 6 on slight pressure.
It would be far safer to use these minerals upon rough gem material than upon cut stones. However, with care and some little skill, hardness tests may be made without particular danger to fine cut material.
The way to proceed is to apply the cut stone (preferably its girdle, or if that is so set as not to be available, a corner where several facets meet) gently to the flat surface of one of the softer test stones, drawing it lightly along the surface and noting the feel and looking to see if a scratch results. If the test stone is scratched try the next harder test stone similarly. Do not attempt to use the test stone upon any valuable cut stone. Proceed as above until the gem meets a test stone that it does not attack. Its hardness is then probably equal to the latter and perhaps if pressed forcibly against it a slight scratch would result, but it is not advisable to resort to heavy pressure. A light touch should be cultivated in this work. Having now an indication as to the hardness of the unknown gem look up those gems of similar hardness in Table 1 and then by the use of some of the tests already given decide which of the stones of that degree of hardness you have. Never rely upon a single test in identifying a gem.
Gemstone Hardness and Detailed Information Links
Alexandrite is a variety of chrysoberyl that displays a color change (alexandrite effect) dependent upon the nature of ambient lighting. The Alexandrite effect is the phenomenon of an observed color change from greenish to reddish with a change in source illumination due physiological response of the human eye in a particular part of the visible spectrum. This color change is independent of any change of hue with viewing direction through the crystal that would arise from pleochroism. Alexandrite results from small scale replacement of aluminium by chromium ions in the crystal structure, which causes intense absorption of light over a narrow range of wavelengths in the yellow region of the spectrum.
Alexandrite from the Ural Mountains in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light.
According to a popular but controversial story, alexandrite was discovered by the Finnish mineralogist Nils Gustaf Nordenskiöld (1792–1866), and named alexandrite in honor of the future Tsar Alexander II of Russia. Nordenskiöld's initial discovery occurred as a result of an examination of a newly found mineral sample he had received from Perovskii, which he identified as emerald at first. The first emerald mine had been opened in 1831.
Alexandrite up to 5 carats (1,000 mg) and larger were traditionally thought to be found only in the Ural Mountains, but have since been found in larger sizes in Brazil. Other deposits are located in India (Andhra Pradesh), Madagascar, and Sri Lanka. Alexandrite in sizes over three carats are very rare.
Some gemstones described as lab-grown (synthetic) alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than synthetic, but are often called Czochralski alexandrite after the process that grows the crystals.
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Almandine, also known incorrectly as almandite, is a species of mineral belonging to the garnet group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet, of deep red color, inclining to purple. It is frequently cut with a convex face, or en cabochon, and is then known as carbuncle. Viewed through the spectroscope in a strong light, it generally shows three characteristic absorption bands.
Almandine is one end-member of a mineral solid solution series, with the other end member being the garnet pyrope. The almandine crystal formula is: Fe3Al2(SiO4)3. Magnesium substitutes for the iron with increasingly pyrope-rich composition.
Almandine, Fe2+3Al2Si3O12, is the ferrous iron end member of the class of garnet minerals representing an important group of rock-forming silicates, which are the main constituents of the Earth's crust, upper mantle and transition zone. Almandine crystallizes in the cubic space group Ia3d, with unit-cell parameter a ≈ 11.512 Å at 100 K.
Almandine is antiferromagnet with the Néel temperature of 7.5 K. It contains two equivalent magnetic sublattices.
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Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.
The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section.
A clear variety first found in Andalusia, Spain can be cut into a gemstone. Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence, although the colors are actually the result of unusually strong pleochroism.
It is associated with mica schist which increases alkali content in ultimate product and so it has not been exploited economically so far.
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Andesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is (Ca, Na)(Al, Si)4O8, where Ca/(Ca + Na) (% Anorthite) is between 30%-50%. The formula may be written as Na0.7-0.5Ca0.3-0.5Al1.3-1.5Si2.7-2.5O8.
The plagioclase feldspars are a continuous solid solution series and as such the accurate identification of individual members requires detailed optical study, chemical analysis or density measurements. Refractive indices and specific gravity increase directly with calcium content.
Be aware that a lot of the andesine on the market today is treated. See this article for details - The Great Andesine Scam
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