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Gemstone Durability and Hardness
The durability of a gemstone is an important consideration when creating jewelry, especially rings. Some rings, such as wedding rings, are worn daily, and are subject to knocks and blows that can damage a gemstone. So which gems are strong enough to be set in a ring that will be worn everyday? And which gems are strong enough for rings that are worn on a more occasional basis? Are there gems which simply shouldn't be set in rings at all?
One important measure of gemstone durability is hardness. In gemology, gem hardness is measured on a scale known as the Mohs scale, which assigns minerals a value between 1 (softest) and 10 (hardest). This scale was invented by a German mineralogist, Frederick Mohs, in 1822. It defines hardness in terms of scratch resistance, where a harder mineral will scratch a softer one, but not vice versa. Diamond is graded the hardest (10), followed by ruby and sapphire (9), topaz, emerald and spinel (8), and garnet, tourmaline and quartz (7). The softest stones include talc (1), gypsum (2), calcite (3) and fluorite (4).
The line between harder and softer gems is generally thought to be a Mohs rating of 7, where gems with a hardness of 7 or more are suitable for rings, while those below 7 are not. But this rule of thumb, while useful, is a bit too simplistic to use alone.
For a ring like a wedding or engagement ring, intended for everyday wear over a period of years, a hardness rating of 10 to 8 is recommended. But other considerations also come into play. Emerald, for example, is typically heavily included with many tiny interior fractures. So despite its hardness rating of 8, it is not really robust enough for a daily wear ring. Topaz is sometimes thought to be disqualified for daily wear as well, because it has perfect cleavage. But diamond also has perfect cleavage and has proved very durable indeed. The risk with perfect cleavage mainly occurs during the cutting process; once such a gem has been cut and set it is unlikely to break. The main strike against topaz is that the blue topaz is common and heavily treated. But the imperial topaz is a rare and fine gemstone that is suitable for a special ring that can be worn every day.
Gems with a hardness of 7 or greater -- particularly quartz, garnet, beryl, zircon and tourmaline -- are quite suitable for rings which are worn more occasionally, or for daily wear rings with protective settings such as a bezel. Gems with a hardness less than 7 can be set in rings, but wear should ideally be restricted to special occasions, and worn with care. Nonethless, some of the softer gems, such as tanzanite and opal, are commonly set in rings.
It is worth mentioning that there are a few gems with a hardness less than 7 which are extremely durable. The special cases are nephrite and jadeite, the two varieties of jade. While nephrite has a hardness of 6 to 6.5, and jadeite a hardness of 6.5 to 7, both are very tough materials due to their internal structures which resist fracturing. Nephrite, in particular, is very strong due to its fibrous crystals which interlock in a matted texture. Jade will often outlast harder gems, though an occasional repolishing may be needed to keep it looking new over the years.