Although durability is, listed in Assignment No. 1 as a prerequisite of a material in order to be considered a gemstone, a study of the variations in hardness and toughness among the important gemstones makes it clear that the durability requirements are not very great. A gemstone may be fairly fragile but very popular. For example, opal is one of the most cherished of stones and certainly one of the most widely sold, yet it is neither hard nor tough. It is among the softest and most fragile, although it is probably slightly harder than the average non gem mineral. However, the important thing is that a gem must be sufficiently resistant to surface wear and breakage to be practical for use in jewelry. What the lower limits are in these two durability categories is hard to say, but it is certain that if the wearer is willing to exercise the care necessary to protect his gemstone from damage, almost any hardness is not too low.
But a similar reasoning, almost any stone that can be cut effectively is tough enough to wear in a protected setting, if it is worn with care. For example, an early graduate of the Institute has worn an opal tie pin for about fifty years without re-polishing and it is as lovely as ever. There are a number of items worn or used for the purpose of beautification that are much less durable than even a fragile gemstone, yet there is certainly a lower limit to the hardness and toughness of a gem that is practical for use in jewelry. This practical lower limit for use in rings is considerably higher than that for use in pins, earrings and other pieces that are likely to receive less abuse.
A common fallacy among jewelers regarding the durability of gemstones is to confuse hardness and toughness. It is believed that a hard stone must be tough and that a stone low in hardness must be fragile. This is far from the truth. For example, although pearls are 4 or less in hardness, they are so tough that many are used in rings. A low hardness means only that the surface will become abraded at on how low the hardness is, plus the care it is given. Actually some gemstones are very hard but fragile. If diamonds are poorly seated in a mounting, with unequal pressure applied by the prongs, cleavage is likely. If a blow is delivered at the right angle, it does not have to be very sharp to cause cleavage. Topaz is notably fragile, for its cleavage is rather easily developed. Zircon and emerald are also quite fragile, for they fracture easily. Consideration of the difference between hardness and toughness makes the relationship clear. If a gem is touch but not hard, its surface may be dulled over a period of time, but it may be re-polished easily. A lack of toughness, on the other hand, threatens the life of a gemstone, since it may be broken during ordinary wear. Low hardness has a greater effect on some types of gem material than it has on others. If a gem is desirable chiefly because of its color an abraded surface has less effect on its beauty than if it is cherished far other attributes that require a fine surface for effectiveness. For example, the jades are not materially affected by a slightly dulled surface, but a moonstone of equal hardness is much less attractive when its surface has become abraded.
Many misstatements are made to the effect that diamond cannot possibly be abraded by other minerals. This disagreement is probably based on the improper use of the terms SCRATCH and ABRADE. It is true that it requires a sharp edge of a diamond to make a visible scratch across the surface of another diamond, however, diamond can be slowly abraded by softer materials. Diamonds used in industry for their extreme hardness are slowly worn down in contact with the much softer materials. For example, diamond dies through which wire is drawn are slowly enlarged by long use for this purpose, and are enlarged to different gauges in successive steps as they become too large for use with the original gauge size with which they started. Diamond tools are also used to true abrasive wheels of silicon carbide. However, diamond so much harder than even silicon carbide that those set in tools used to true abrasive wheels often last long enough to true thousands of such Wheels.
The most obvious dividing line between the hard and soft gemstones is at 7 on Mohs' scale. The reason for this is the fact that quartz (hardness 7) is the most common of minerals and is an important constituent of ordinary dust, as a result, the simple act of wiping the dust from a gem of 7 or softer, causes an accumulation of minute abrasions that will eventually dull the surface. This suggests that dust should be brushed lightly from the surface of such a gem or blown from it, not wiped off. However, even those stones on which the surface has become dulled by abrasion can be re-polished at low cost.