Smoky Quartz

A smoky-gray, brown to black variety of quartz that owes its color to gamma irradiation and the presence of traces of aluminum built into its crystal lattice (Griffiths et al, 1954; O'Brien, 1955). The irradiation causes the aluminum Al(+3) atoms that replace Si(+4) in the lattice in a [AlO₄]^- group to transfer an electron to a neighboring monovalent cation (often Li⁺) and form a [AlO₄/Metal⁰]⁰ color center (O'Brien, 1955).
The (German) name Morion is used for black smoky quartz.

Smoky quartz is dichroic (from darker yellow-brown to lighter red-brown) when viewed in polarized light. The photo to the left shows the change of color in a smoky quartz crystal that is rotated in front of an LCD display that serves as a source of polarized light.
The color of smoky quartz is sensitive to heat and will pale at temperatures above 200-300°C or by prolonged exposure to UV light. This loss of color can be reverted by gamma irradiation of the crystals.

Smoky quartz can be found in many different environments but is most characteristic for pockets in igneous rocks and pegmatites. The quartz grains in granites and related rocks are often smoky.
Smoky quartz shows many different habits and crystallographic forms, but needle-like crystals of smoky color are not known.
Crystals that grew at relatively high temperatures, in particular macromosaic quartz crystals like those found in pegmatites and alpine-type fissures are often evenly colored, whereas crystals from other environments often show a color zonation in the form of multiple phantoms. In scepters and skeleton quartz, the color may be more intense along the edges of the crystal.

Note: Very often black or brown crystals that are colored by inclusions of minerals or organic matter are erroneously called "smoky quartz" or "morion". Typical examples of such misnomers are black quartz crystals embedded in sedimentary rocks, like those found in gypsum, anhydrite, and limestone in Italy and Spain. True smoky quartz can be distinguished from crystals that are colored by inclusions by its dichroism.