Smoky Quartz on Microcline
Sheep's Head Area, Colorado

This specimen measures 12cm x 21cm


Quartz      SiO2

Silicates; Silicon dioxide

Color and Luster: White, colorless, and in various shades as follows: rockcrystal, transparent, colorless; amethyst, purple, rose quartz, pink, rose-red; citrine, clear yellow; smoky quartz, plae brown to black; milky quartz, milk white; aventurine, glistening with enclosed scales of mica or hematite; cat's eye, opalescent from inclusions of asbestos; and tigereye, with lusterous yellow to brown parallel fibers; vitreous, greasy luster; streak white

Hardness: 7

Cleavage: Generally none; rarely indistinct rhombohedral parting

Fracture: Conchoidal

Crystals: Hexagonal; usually prismatic crystals striated crosswise and frequently terminated by double rhomohedrons(like hexagonal pyramids); also granular, disseminated, massive


Orthoclase     KAlSi3O8

Tectosilicate; potassium aluminum silicate, sometimes with considerable amounts of sodium

Color and Luster: White, pink, brown, gray, green, colorless, yellowish; vitreous, streak white

Hardness: 6-6.5

Cleavage: Good, two directions at 90°

Fracture: Uneven

Crystals: Monoclinic or triclinic; predominantly as single crystals of rectangular or square cross section and as tabular crystals; twin crystals very common; also cleavage masses, granular, disseminated grains

Note: Orthoclase, sanadine, adularia, and microcline are the potash feldspars Andularia is often called moonstone, with a transparent or opalescent appearance Sanadine is glassy Microcline is white to pale yellow, rarely red, and when bright green it is called amazonite

Photo by Mary Sutherland


Descriptions taken from
The Audubon Society Field Guide to North American Rocks and Minerals