The gems we prize most are very rare. We find them only in small deposits in a few places. Even gems you might consider common, like amethyst and chalcedony, are rare compared to the ordinary rocks that make up the Earth's crust. You might find plenty of quartz pebbles hiking on a mountain trail, but you would be lucky to find any of gem quality.
Why are gems rare? What makes them so much harder to find, and thus more valuable, than other kinds of minerals? The answers lie in the special environment, the unique combination of conditions, that must exist before a gemstone can form in nature. There are 3 Major factors that make up these unique environments:
Correct combination of Raw Materials
The different raw materials available in a gem-forming environment have the most obvious bearing on what gems may form there. Take quartz, for example. Its chemical formula, SiO2, tells us quartz forms where silicon (Si) and oxygen (0) are available. Since these two elements make up over 74 percent of the weight of the Earth's crust, it is not surprising that we find lots of quartz everywhere in the world-though seldom in gem quality.
In contrast, spodumene, the mineral species that gives us the gem kunzite, is much rarer than quartz. A look at its chemical formula, LiAlSi06, tells us why. To cook up some spodumene we need an environment where the rare element lithium (Li) is available along with aluminum (Al) and the even more common silicon and oxygen. There are tough odds against all four of these elements' coming together by accident. Aluminum weighs in at eight percent of the Earth's crust, but lithium makes up less than one hundredth of one percent!
Correct Raw Materials
Correct Temerature & Pressure
If we hoped to have spodumene form in the characteristic pink gem crystals of kunzite, we would also need to have some manganese in the environment. This is even more unlikely to happen, since the Earth's crust is only one tenth of one per cent manganese by weight.
As the case of kunzite shows, the chance availability of an extra ingredient often makes a big difference in the value we place on gems that form in a given environment. Sapphires and rubies are another good example. Both are varieties of corundum (AI203), a mineral that forms as a colorless crystal where aluminum and oxygen are available at the right conditions of temperature and pressure. But only if small amounts of iron and titanium are also available in the same environment is there a chance of corundum forming as blue sapphire. And before ruby can form, there must be some chromium available.
Even in those rare cases where an environment contains all the right raw materials needed to form a gem, the temperature and the pressure conditions must also be in the right balance. Otherwise, instead of a gem, you can get something quite different.
Look at what happens with the element carbon, for example. In some combinations of heat and pressure, carbon (in the presence of oxygen) burns up to produce carbon monoxide (CO) or carbon dioxide (C02) gas. In different temperature and pressure conditions, carbon does crystallize-but in the form of the soft, black, flaky mineral known as graphite, used in pencil leads. Only in the rare combinations of extreme heat and pressure found between 75 to 120 miles beneath the Earth's surface will carbon naturally crystallize as diamond.