Photo: Rocks4passion


Quartz may be the second most common mineral on our planet’s crust, but everyone from the ancient Egyptians and Australian Aborigines to modern-day New Age practitioners believe that this crystal holds mystical or healing properties.

Black quartz is a variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque black crystal.

Like other quartz gems, it is a silicon dioxide crystal. The smoky colour results from free silicon formed from the silicon dioxide by natural irradiation.

Color

The color of smoky quartz is caused by irradiation and traces of aluminum built into its crystal lattice. Aluminum replaces silicon to form a [AlO4]- group instead of [SiO4]. To compensate for the imbalance of charge in the lattice, small monovalent cations (H+, Li+ or Na+) are built into the lattice, as well. High energy radiation transfers the extra electron from [AlO4]- to the cation, and a color center is formed. Interestingly, H+ seems to interfere with this process, and higher concentrations of built-in hydrogen inhibit the formation of color-centers. In normal geological environments this process can only take place at temperatures below 50°C, otherwise the rate of color center destruction surpasses that of color center formation. So the color of the 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 in a granite of average composition.



True smoky quartz will lose its color when heated to about 200°C, and the color will occur again when the crystal is irradiated with x- or gamma-rays.

It's still common practice to artificially irradiate colorless quartz and sell it as smoky quartz.

Is irradiated smoky quartz safe?

There is no radiation hazard associated with irradiated smoky quartz. Irradiating a crystal will cause some energy to be stored in the crystal structure, which can change the optical properties, typically observed as a change in color.

Occurrence

Smoky quartz is usually found in intrusive igneous and certain high grade metamorphic rocks, like granite and orthogneiss, as these contain traces of radioactive elements whose radiation cause the coloration.
Smoky quartz from volcanic rocks is more uncommon, here amethyst and colorless quartz dominate.

Smoky quartz from sedimentary rocks is very unusual. Their content of radioactive elements is usually very low. Much of the "smoky" quartz reported from sedimentary rocks is actually not true smoky quartz, but quartz containing black or brown inclusions.

Varieties

Cairngorm: Synonym if Smoky Quartz. This term is used mostly in Scotland, specifically for the deposit in the Cairngorm Mountains where it is found.
Morion:  Morion is a very dark brown to black opaque variety. Morion is the German, Danish, Spanish and Polish synonym for smoky quartz.

Morion, a variety of smoky quartz
Photo: Dario Crespi

See also:
How Is Aura Rainbow Quartz Made?
What Causes the Pink Color of Rose Quartz?

 
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