Crystal, or drusy, formation on rocks
When a precious stone has tiny crystals formed on its surface, it is said to be drusy. When mineral and silica rich water gets into the porous area of a gem, it eventually cools and crystallization occurs, and this is how drusy is formed. Initially, drusy was merely the object of curiosity for gemologists and like-minded enthusiasts. In recent years, however, it has garnered mainstream and commercial popularity. It's properties are that of quartz.
Makeup
Silicon dioxide is one part silicon and two parts oxygen. It is the most abundant mineral in the earth's crust, and is used in everything from glass making to fiber optic technology.
Hardness
Quartz has a rank of seven on the Mohs scale of hardness. The scale was created by German Friedrich Mohs in 1812. Quartz's hardness makes it versatile, and further increases its popularity and usage. Quartz has chemical properties that make it resistant to heat and chemicals, all of which makes drusy that much more durable.
Color
Quartz appears in just about every color. Quartz on drusy, however, is clear. Popular drusy types are Malachite (green), Psilomelane (black) and Cobalto Calcite (pink). The aforementioned colors are from the stone, not the quartz since its diaphaneity is clear, which is to say that you are able to see through it.
Luster
Quartz has a vitreous luster, meaning that it is of a glassy quality. This enhances the aesthetic appeal of drusy, given that the natural color of the gem is enhanced, not masked, by the quartz.
Quality
With drusy stones, there are several determining factors to look at when it comes to its quality. To begin, there are the usual criteria that normal gems are measured by: cut, color, carat weight and clarity. With drusy, however, there is also a coverage issue. The more evenly dispersed the quartz crystal is over the stone, the better the value. Perfect crystal distribution is rare, since the pores on the stone that the mineral-rich water seeped into are not perfectly spaced. In addition, the quartz may have developed more in some places than others.
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