Hardness is a gem's ability to resist scratching of its surface. This property derives from the crystal structure of the gem in virtue of how densely the atoms are packed, and how strong the binding forces between them are. It not only affects durability, but also has implications for the potential luster of the polished gem, and dictates what sort of tools, abrasives and polishes the lapidary requires to work with it.
Most everyone has run across the Mohs' Scale of hardness which ranks materials in a kind of pecking order from 1 at the low end (can't scratch anything other than itself) to 10 at the high end (scratches all other gems, including itself). This scale was set up by the Austrian mineralogist, Friedrich Mohs, in 1827. He chose talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond as the representatives of hardnesses 1-10, respectively.
Mineralogists and geologists can determine the approximate hardness of their specimens by using a set of "hardness points" which are metal pens set with pointed tips made of these minerals. The unknown specimen's hardness is determined by sequentially using the points until the one which will not scratch the specimen is found. So, if hardness point #8 will scratch the item, but #7 will not, then its hardness lies between 7 and 8.
A kind of field, or practical, version of this test is often used by rockhounds and amateur geologists:
1, 2 (VERY EASILY AND EASILY SCRATCHED BY FINGERNAIL)
3,4 (VERY EASILY AND EASILY SCRATCHED BY COPPER COIN)
5,6 (VERY EASILY AND EASILY SCRATCHED POCKET KNIFE BLADE)
7 (SCRATCHES WINDOW GLASS, SCRATCHED BY STEEL FILE)
8 - 10 (SCRATCHES WINDOW GLASS, NOT SCRATCHED BY STEEL FILE)
The numbers on this simple and useful scale are sometimes misunderstood to be linear or proportional in their meaning, which is not true. In order to get precise determinations of hardness a device called a sclerometer is used. It pushes a diamond point into a surface and measures the exact force needed for penetration. This type of test belies our feeling that apatite (4 on the Mohs' Scale), must be about half as hard as topaz (8 on the Mohs' Scale). Sclerometer readings show that a topaz gem requires 8.5 times the force to scratch as does an apatite. For corundum (9) and diamond (10) the difference is even more striking--> with diamond testing as 140x harder than sapphire.
Soft Gems [Ivory & jet = 2.5, pearl = 3, sphalerite = 3.5, fluorite = 4] Intermediate Gems [Scapolite = 6, Tanzanite = 6.5, garnet = 7. - 7.5, tourmaline = 7.5] Hard Gems [Spinel = 8, topaz = 8, chrysoberyl = 8.5, corundum = 9]
Hardness can vary with crystal direction. The most famous example of this phenomenon is the gem kyanite with dual hardnesses of 5 and 7, depending on direction. Going with the "weakest link" idea, we are well advised to treat kyanite as a relatively soft gem. Lapidaries working with this gem have to constantly adjust their pressure and speed so that progress is made on the harder areas, yet softer areas are not overcut.
[Kyanite: H = 5 & 7]
Interestingly, if diamond crystals did not vary in hardness with direction, they couldn't be cut and polished with diamond abrasives. The diamond cutting process uses a slurry of tiny crystals of natural or synthetic "bort" (industrial grade diamond) on a spinning cast iron surface (lap). As the various facets of the diamond are cut and polished, they are subjected to these randomly oriented crystals, at least some of which have harder surfaces exposed than the facet being cut. The hardest crystal direction of a diamond is the "octahedral" face which, literally, cannot be polished. Part of the job of the diamond cutter, then, is to orient the rough to avoid this plane in any of the facets.
Even with the variable hardness factor, diamond cutting is time consuming. It requires specialized equipment, capable of greater rotational speed of the cutting wheel and greater pressure on the gem than does equipment used to cut colored stones. Although there are a few cutters who have the skill and equipment necessary to work both with diamonds and colored stones, the vast majority specialize in one or the other.
Although hardness is an important characteristic in a gem, it is by no means the final measure of a gem's wearability or suitability for a particular use. All other factors being equal, the harder the gem, the better it will wear. But, there are two other factors which can make all the difference in the world: a gem's toughness and its stability.
Each of these attributes will be treated in subsequent essays, but for now, a brief synopsis. Toughness is the ability to resist breaking or chipping, and is an extremely important consideration when selecting a gem for, say, an engagement ring.
A hard gem will retain its polish, but if it is not tough, it may chip or break. A notable example is topaz. With hardness 8, it might seem ideal for an everyday ring or bracelet, but it is, in fact, a rather fragile gem due to its tendency to cleave (break cleanly along a cyrstal plane). Topaz though unlikely to scratch, may chip or break with hard, constant wear.
On the other hand, the jades with a hardness between 6 and 6.5 might seem poor candidates for heavy wear, but truth be told, they are the toughest of all natural gem materials and wear like iron! Their seemingly contradictory historical uses, both in the most intricate and delicate carvings, and as workaday tools such as axes, is testament to this durability.
Stability meaures a gem's ability to resist changes due to light, chemicals and heat. It's little comfort to have a hard and/or tough gem, if it can be altered by absorbing chemicals from the air or from your skin, like pearls, or if its color changes due to light exposure like brown topaz.
In general, jewelry that is worn more than occasionally should be set with gems of at least hardness 7 and that have good toughness and stability. Softer, more fragile, and less stable gems can be enjoyed in jewelry that has protective settings or which is worn gently. Extremely delicate gems are best kept as collectors' objects.