Smoky quartz, one of the few dark colored semi-precious gems, is formed when normal quartz is subjected to long-term radiation.
More Info: Smoky quartz is typically found in granite mountains that have small but persistent levels of radioactivity. Smoky quartz usually contains aluminum impurities that turn the quartz smoky gray when exposed to radioactivity. If enough aluminum is present, the quartz can turn completely black, and is called "morian". When treated with extremely high temperatures, the quartz will return to a clear form.
A Closer Look
Two thousand years ago, a Roman scholar known as Pliny the Elder suggested that quartz might be permanently frozen ice, because it was often found near glaciers. His idea remained popular until the scientific revolution in the eighteenth century. Quartz is actually silicon dioxide, a combination of silicon and oxygen, the two most abundant elements in earth's crust, and is found in virtually every type of known rock type, including igneous, sedimentary, metamorphic, meteorites, and lunar rocks. Pure silicon dioxide is clear, but impurities can cause it to be many different colors. The various colors of quartz are often given different names: amethyst (purple color - iron and magnesium impurities), rose (red or pink - thallium or titanium impurity), and citrine (yellow - iron impurity).
Smoky quartz is found naturally in Brazil, Colorado, Scotland, and the Swiss Alps, and is among the rarest of semi-precious gems. Because it is so rare, some gem dealers will intentionally irradiate normal quartz to artificially create smoky quartz.