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 POCKETKNIFE 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 as hard as 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 hard 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 -- so part of the job of the diamond cutter 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 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 chips readily. On the other hand, jade with a hardness between 6 and 6.5 might seem a poor candidate for heavy wear, but truth be told, it is the toughest of all natural gem materials and wears like iron! Its seemingly contradictory historical uses both in the most intricate and delicate carvings, and as workaday tools such as axes, is testament to its 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 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 that have relatively high 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.