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■ Quartz Crystal with Ghost
The black area in this colorless quartz crystal shows the “ghost” or shape of
the crystal at earlier stages of growth. The dark material was mixed into the
crystal as it grew.Tiny bits of carbon, sulfide minerals, or tiny dark tourmaline
crystals can color all or part of a quartz crystal dark gray or black. This is not
the same as smoky quartz in which dark colors are produced by crystal
defects activated by radiation.

■ Amethyst Amethyst
Amethyst quartz, such as this specimen from
Guanajuato, Mexico, is colored purple or violet
by iron impurities in its crystal structure.The purple
turns darker when amethyst is exposed to radiation.
Heating can cause amethyst to become colorless or
yellow-brown.

Gift from the Geology Museum, Colorado School of Mines

■ Colemanite
Colemanite was first identified as a distinct mineral in 1882 in
the Death Valley borate deposits. Furnace Creek and the Death
Valley flats have been mined for colemanite and borax for more than a hun-
dred years. Colemanite is used to make glass and is a source of boron, whose
compounds are used in atomic energy. Colemanite is named for William T.
Coleman, a San Francisco merchant and mine owner.
Large specimen collected by Modesto Leonardi and donated by the Department of Geology, California Academy of Sciences,

San Francisco, courtesy of Charles W. Chesterman. Smaller specimens are gifts from Robert Countryman, Tenneco Oil Co.,

Lathrop Wells, Nevada.

■ Pyrite Crystals
Pyrite often crystallizes in cubes, octahedrons, and pyritohedrons with stri-
ated crystal faces. Pyrite cubes up to 15 cm (6 in) across have been found in
mines near Leadville, Colorado.This specimen is from Bingham, Utah. Pyrite
also forms nodules and masses.

Pyrite is burned to produced sulfur dioxide and sulfuric acid. Pyrite struck
with steel will produce sparks, and large masses of pyrite have spontaneously
combusted in mines. Its name comes from the Greek pyr (“fire”).

Gift from the Geology Museum, Colorado School of Mines, Golden, Colorado.

■ Scalenohedral Calcite Crystals
Calcite forms crystals with hexagonal symmetry, but also forms in tabular
crystals, needlelike crystals, and in masses or grains that do not exhibit crys-
talline shapes. Some calcite crystals are very large; crystals weighing 450 kg
(1000 lbs) have been found in a limestone cave in NewYork.This specimen is
from Terlingua, Texas.

Gift from the Geology Museum, Colorado School of Mines, Golden, Colorado. Collected by J. Harlan Johnson.

■ Fossil Ammonite Impression
The patterned form pressed into this sandstone was made by the shell of an
ammonite. Ammonites are a group of extinct marine animals that lived from
the Paleozoic more than 400 million years ago until the close of the Creta-
ceous Period about 65 million years ago. Their soft, tentacled bodies were
enclosed in a shell. The shell was divided into chambers that made intricate
curved patterns on the shell.

Gift from Harrison Cobb, Boulder, Colorado.

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