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You are here: Physical > Hardness

Gemstone Hardness

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
Hardness Material Absolute Hardness
10 Diamond  1500
9 Corundum  400
8 Topaz  200
7 Quartz  100
6 Othoclase  72
5 Apatite  48
4 Fluorite  21
3 Calcite  9
2 Gypsum  3
1 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:

  • 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

Gem Hardness More Information
Agate 6.5 - 7 Agate Information
Alexandrite 8.5 Alexandrite Information
Almandine Garnet 6.5 - 7.5 Almandine Garnet Information
Amazonite 6 - 6.5 Amazonite Information
Amber 2 - 2.5 Amber Information
Amethyst 7 Amethyst Information
Ametrine 7 Ametrine Information
Andalusite 7.5 Andalusite Information
Andesine 6 - 6.5 Andesine Information
Apatite 5 Apatite Information
Aquamarine 7.5 - 8 Aquamarine Information
Aventurine 7 Aventurine 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
Chrysoberyl 8.5 Chrysoberyl Information
Chrysocolla 2 - 4 Chrysocolla Information
Chrysoprase 6.5 - 7 Chrysoprase Information
Citrine 7 Citrine Information
Clinohumite 6 Clinohumite Information
Demantoid Garnet 7 - 7.5 Demantoid Garnet Information
Diamond 10 Diamond Information
Emerald 7.5 - 8 Emerald Information
Fluorite 4 Fluorite Information
Gaspeite 4.5 - 5 Gaspeite Information
Grossularite Garnet 6.5 - 7.5 Grossularite Garnet Information
Hematite 5.5 - 6.5 Hematite Information
Hemimorphite 5 Hemimorphite Information
Hessonite Garnet 6.5 - 7.5 Hessonite Garnet Information
Hiddenite 6.5 - 7 Hiddenite Information
Idocrase 6.5 Idocrase Information
Iolite 7 - 7.5 Iolite Information
Jadeite 6 - 6.5 Jadeite Information
Jasper 6.5 - 7 Jasper Information
Kunzite 7 Kunzite 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
Quartz 7 Quartz Information
Rhodochrosite 4 Rhodoschrosite Information
Rhodolite Garnet 6.5 - 7.5 Rhodolite Garnet Information
Rose Quartz 7 Rose Quartz Information
Ruby 9 Ruby Information
Ruby-Zoisite 6.5 - 7 Ruby Zoisite Information
Sapphire 9 Sapphire 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
Spinel 8 Spinel 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
Topaz 8 Topaz Information
Tourmaline 7 - 7.5 Tourmaline Information
Tsavorite Garnet 6.5 - 7.5 Tsavorite Garnet Information
Turquoise 5 - 6 Turquoise Information
Verdite 3 Verdite Information
Zircon 6.5 - 7.5 Zircon Information