Smoky Quartz gemstone color formation

Smoky Quartz is a variety of Quartz, it is a silicon dioxide mineral ranging in color from pale, smoky gray to deep brown and black, sometimes yellowish-brown. Though Smoky Quartz can be nearly opaque, it is almost always transparent to translucent.  Smoky Quartz offers a rare and beautiful transparency that draws light into shades of black and brown. Smoky Quartz was known as a Stone of Power.  

The color of smoky quartz is caused by irradiation and aluminum impurities built into its crystal lattice to form the color-centers [1].  In normal geological environments, the color centers formation process can only take place at temperatures below 50°C, otherwise the rate of color center destruction surpasses that of color center formation.  Thus the color of smoky quartz crystals appeared long after the crystals have grown. It is estimated that it takes several million years for a crystal to assume a deep color by natural radiation emitted in nearby rocks [2] [3].

The color of smoky quartz is typically much more homogeneous than that of amethyst, but often still more intense at the tips of the crystals. Natural smoky quartz will lose its color when heated to about 200°C, and the color will occur again when the crystal is irradiated with gamma-rays. Some smoky quartz is sensitive to ultraviolet light and will pale when exposed to sunlight for a long time.

At some locations smoky quartz shows zones of violet or yellow color along with smoky ones. The different colors might be found in different layers as "phantoms", or they show a patchy distribution in one crystal.  It is also found Smoky Amethyst Quartz - a combination of Amethyst and Smoky Quartz and Smoky Citrine Quartz - a combination of Citrine and Smoky Quartz.

To artificially irradiate colorless quartz can produce smoky quartz. It's impossible to tell whether the smoky quartz is natural or not just by looking at the crystals. However, natural Smoky Quartz tends to be smoky to the base, whereas irradiated tends to have a white crystal base next to the matrix rock.  Also if it is so black you cannot see through it and it has good surface luster, it is probably irradiated. 

Smoky quartz shows a change of color from yellow-brown to red-brown when turned around an a-axis in polarized light. Like all crystal quartzes, the Smoky quartz has a hardness of 7 on the Mohs scale and is thus insensitive to scratches.

Smoky Quartz is found throughout the world.  It is usually found in intrusive igneous and certain high grade metamorphic rocks as these contain traces of radioactive elements whose radiation causes the coloration. Important deposits exist in Brazil, Madagascar, Mozambique, Australia, Switzerland, Scotland, and the United States (Colorado and New Hampshire).

Smoky Quartz is an affordable gemstone, and is used in various form of gemstone beads jewelry. Smoky quartz is very common and often lacking flaws, it can reach several hundred carats with minimal flaws. High quality Smoky Quartz is usually faceted for ring and pendant centerpieces, and less quality Smoky Quartz is most often used in beads with different shapes:  round, coin, oval, square, pillow, cube, tube, et al. With Smoky Quartz beads in your favorite color and shape, you even can create your own unique piece of Smoky Quartz jewelry, and with plenty of opportunity to customize to fit your unique style. There are plenty of low-cost Smoky Quartz beads available through online store and be sure to check out the discounted Smoky Quartz beads first to start your design.

The fact that Smoky Quartz can lose its color makes it obvious that Smoky Quartz jewelry should not be worn while sunbathing and high temperature can also be harmful to the Smoky Quartz.

Reference:

[1] G.R. Rossman, “Colored varieties of the silica minerals”, Reviews in Mineralogy, Vol.29 

[2] Chudoba, K. F. (1962): Some relations between the causes of amethyst, smoky quartz, and citrine colors as given by modern science. Mineralogicheskii Sbornik (Lvov), (16), 91-105.

[3] Cohen, Alvin J. (1989): New data on the cause of smoky and amethystine color in quartz. Mineralogical Record 20, 365-367.