Smoky Quartz: The Black Variety of Quartz

Smoky quartz is a type of quartz crystal that is known for its smoky brown or gray color. It is formed when natural radiation, emitted from the surrounding rock, activates color centers around aluminum impurities within the crystalline quartz. Smoky quartz is a very common gemstone, and it is found in many locations around the world.

The color of smoky quartz is variable from brown to black and sometimes smoky gray colored specimens are included as smoky quartz.

Smoky quartz, very common coarse-grained variety of the silica mineral quartz that ranges in colour from nearly black through smoky brown. 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.

Smoky Quartz
Smoky Quartz, the Black or Brown Variety of Quartz.
Photo: Rocks4passion

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.

Smoky Quartz 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.

Smoky Quartz 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 of Smoky quartz

Varieties Smoky quartz, a variety itself of quartz, has a few varieties of its own.

Smoky Quartz: The most prevalent variety, encompassing a spectrum of gray to brown tones. Translucent to transparent, it exhibits dichroism, appearing darker when viewed along specific crystallographic axes.

Cairngorm: Originating from the Cairngorm Mountains of Scotland, this variety showcases a characteristic smoky yellow-brown to greyish-brown coloration. Typically transparent to translucent, it possesses a vitreous luster and excellent clarity.

Morion: An opaque, nearly black variety, often referred to as "black quartz." The German term "Morion" is believed to stem from a misreading of "mormorion," a type of helmet. Its deep color arises from a higher concentration of color centers within the crystal lattice.

Smoky Citrine: A rare variety exhibiting a blend of smoky brown and yellow-orange hues. The yellow-orange coloration often results from natural or heat-induced conversion of aluminum-bearing color centers.

Smoky Amethyst: This variety displays a combination of smoky brown and amethyst's characteristic purple tones. The color intensity and distribution vary, creating a range of shades from smoky lavender to deep plum with smoky undertones.

Coon Tail Quartz: An uncommon variety characterized by alternating bands of black and gray within the crystal structure. The banded pattern resembles a raccoon's tail, hence its name. This formation arises from variations in irradiation exposure during crystal growth.

Smoky Quartz crystals
Morion, a variety of smoky quartz
Photo: Dario Crespi

Smoky quartz varieties showcase the fascinating interplay between geological processes and their observable effects on mineral properties. Each variation possesses unique characteristics and a distinct geological story, highlighting the multifaceted nature of mineral diversity.

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

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