Smoky quartz is a macrocrystalline variety of the mineral Quartz (SiO2). Quartz is the most abundant single mineral on earth. It makes up about 12% of the earth's crust, occurring in a wide variety of igneous, metamorphic and sedimentary rocks.

Quartz varieties are commonly separated into two groups based on the size of the individual grains or crystals; macrocrystalline quartz in which individual crystals are distinguishable with the naked eye, and cryptocrystalline quartz in which the individual crystals are too small to be easily distinguishable under the light microscope.

Some of the macrocrystalline quartz varieties are: Amethyst, Ametrine, Cat's-eye Quartz, Citrine, Phantom Quartz, Rock Crystal, Rose Quartz, Rutilated Quartz and Smoky Quartz.

Blue Aventurine Quartz and Green Aventurine Quartz are actually quartzites (a rock, not a mineral) composed essentially of interlocking macrocrystalline quartz grains with disseminated grains of other color imparting minerals.

The cryptocrystalline varieties of quartz may be separated into two types; fibrous and microgranular. Chalcedony is the general term applied to the fibrous cryptocrystalline varieties. Agate is an example of a fibrous cryptocystalline banded chalcedony variety of quartz. Carnelian, Chrysoprase and bloodstone are other chalcedony varieties.

Chert is the general term applied to the granular cryptocrystalline varieties of quartz, of which flint and Jasper are examples.

PHYSICAL PROPERTIES
Smoky Quartz chemical composition: SiO2
Class: tectosilicate
Crystal system: Hexagonal-Rhombohedral; 32 (trigonal-trapezohedral)
Crystal habit: Macroscopic crystals commonly occur as horizontally striated hexagonal prisms terminated by a combination of positive and negative rhombohedrons forming six sided pyramids. Prism faces and/or rhombohedral terminations may be lacking, poorly developed or predominant resulting in diverse possible crystal habits. Trigonal tapezohedral faces can occur in the upper right or left of alternating prism faces identifying right or left handed crystals, respectively.

Twinning: Dauphine twin with c the twin axis, Brazil twin with {1120} the twin plane, Japanese twin is rare with {1122} the twin plane.
Specific gravity: 2.65
Index of refraction: 1.54-1.55
Birefringence: maximum of 0.009
Pleochroism: distinct
Hardness: 7
Color: brown to gray to black
Luster: vitreous
Transparency: transparent
Cleavage: none
Fracture: conchoidal
Streak: white