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.
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:
These five test stones represent the following degrees of hardness:
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
|Agate||6.5 - 7||Agate Information|
|Almandine Garnet||6.5 - 7.5||Almandine Garnet Information|
|Amazonite||6 - 6.5||Amazonite Information|
|Amber||2 - 2.5||Amber Information|
|Andesine||6 - 6.5||Andesine Information|
|Aquamarine||7.5 - 8||Aquamarine Information|
|Axinite||6.5 - 7||Axinite Information|
|Beryl||7.5 - 8||Beryl Information|
|Bloodstone||6.5 - 7||Bloodstone Information|
|Carnelian||6.5 - 7||Carnelian Information|
|Chalcedony||6.5 - 7||Chalcedony Information|
|Charoite||4.5 - 5||Charoite Information|
|Chrome Diopside||5 - 6||Chrome Diopside Information|
|Chrysocolla||2 - 4||Chrysocolla Information|
|Chrysoprase||6.5 - 7||Chrysoprase Information|
|Demantoid Garnet||7 - 7.5||Demantoid Garnet Information|
|Emerald||7.5 - 8||Emerald Information|
|Gaspeite||4.5 - 5||Gaspeite Information|
|Grossularite Garnet||6.5 - 7.5||Grossularite Garnet Information|
|Hematite||5.5 - 6.5||Hematite Information|
|Hessonite Garnet||6.5 - 7.5||Hessonite Garnet Information|
|Hiddenite||6.5 - 7||Hiddenite Information|
|Iolite||7 - 7.5||Iolite Information|
|Jadeite||6 - 6.5||Jadeite Information|
|Jasper||6.5 - 7||Jasper Information|
|Kyanite||6 - 7||Kyanite Information|
|Labradorite||6 - 6.5||Labradorite Information|
|Lapis Lazuli||5 - 6||Lapis Lazuli Information|
|Larimar||4.5 - 5||Larimar Information|
|Lepidolite||2.5 - 3||Lepidolite Information|
|Malachite||3.5 - 4||Malachite Information|
|Maw-Sit-Sit||6 - 7||Maw-Sit-Sit Information|
|Melanite||6.5 - 7||Melanite Information|
|Moonstone||6 - 6.5||Moonstone Information|
|Morganite||7.5 - 8||Morganite Information|
|Nephrite Jade||6 - 6.5||Nephrite Jade Information|
|Nuumite||5.5 - 6||Nuumite Information|
|Obsidian||5 - 5.5||Obsidian Information|
|Onyx||6.5 - 7||Onyx Information|
|Opal||5.5 - 6.5||Opal Information|
|Orthoclase||6 - 6.5||Orthoclase Information|
|Pearl||2.5 - 4.5||Pearl Information|
|Peridot||6.5 - 7||Peridot Information|
|Prehnite||6 - 6.5||Prehnite Information|
|Pyrope Garnet||6.5 - 7.5||Pyrope Garnet Information|
|Rhodolite Garnet||6.5 - 7.5||Rhodolite Garnet Information|
|Rose Quartz||7||Rose Quartz Information|
|Ruby-Zoisite||6.5 - 7||Ruby Zoisite Information|
|Scapolite||5.5 - 6||Scapolite Information|
|Seraphinite||2 - 4||Seraphinite Information|
|Sodalite||5.5 - 6||Sodalite Information|
|Spessartite Garnet||6.5 - 7.5||Spessartite Garnet Information|
|Sphalerite||3.5 - 4||Sphalerite Information|
|Sphene||5 - 5.5||Sphene Information|
|Spodumene||6.5 - 7||Spodumene Information|
|Sugilite||6 - 6.5||Sugilite Information|
|Sunstone||6 - 6.5||Sunstone Information|
|Tanzanite||6.5 - 7||Tanzanite Information|
|Tiger's Eye||6.5 - 7||Tiger's Eye Information|
|Tourmaline||7 - 7.5||Tourmaline Information|
|Tsavorite Garnet||6.5 - 7.5||Tsavorite Garnet Information|
|Turquoise||5 - 6||Turquoise Information|
|Zircon||6.5 - 7.5||Zircon Information|