Amethyst 24 VISIT OR GUIDE Quar tz Crystal with Ghost The blac k area in this colorless quar tz crystal shows the ghost or shape of the crystal at earlier stages of growth The dark material w as mixed into the crystal as it grew T in y bits of carbon sulfde minerals or tin y dark tourmaline crystals can color all or par t of a quar tz crystal dark gray or blac k This is not the same as smok y quar tz in whic h dark color s are produced by crystal defects acti vated by radiation Ameth yst Ameth yst quar tz suc h as this specimen from Guanajuato Mexico is colored purple or violet by iron impurities in its crystal structure The purple turns dark er when ameth yst is exposed to radiation Heating can cause ameth yst to become colorless or yellow brown Gift from the Geolog y Museum Colorado School of Mines Colemanit e Colemanite w as fr st identifed as a distinct mineral in 1 882 in the Death V alley borate deposits F urnace Creek and the Death V alley fats have been mined for colemanite and borax for more than a hun dred year s Colemanite is used to mak e glass and is a source of boron whose compounds are used in atomic energy Colemanite is named for William T Coleman a S an F rancisco merc hant and mine owner Large specimen collected b y Modesto Leonardi and donated b y the Department of Geolog y California Academ y of Sciences San F rancisco courtesy of Charles W Chester man Smaller specimens are gifts from Robert Countryman T enneco Oil Co Lathrop W ells Ne v ada Pyr it e Crystals Pyrite of ten crystalliz es in cubes octahedrons and pyritohedrons with stri ated crystal faces Pyrite cubes up to 1 5 cm 6 in across have been found in mines near L eadville Colorado This specimen is from Bingham Utah Pyrite also forms nodules and masses Pyrite is burned to produced sulfur dio xide and sulfuric acid Pyrite struc k with steel will produce sparks and large masses of pyrite have spontaneously combusted in mines Its name comes from the Greek pyr fre Gift from the Geolog y Museum Colorado School of Mines Golden Colorado Scalenohedr al Calcit e Crystals Calcite forms crystals with hexagonal symmetry but also forms in tabular crystals needlelik e crystals and in masses or grains that do not exhibit crys talline shapes S ome calcite crystals are very large crystals weighing 450 k g 1 0 0 0 lbs have been found in a limestone cave in New Y ork This specimen is from T erlingua T exas Gift from the Geolog y Museum Colorado School of Mines Golden Colorado Collected b y J Harlan Johnson F ossil Ammonit e Impr ession The pat terned form pressed into this sandstone w as made by the shell of an ammonite Ammonites are a group of extinct marine animals that li ved from the P aleoz oic more than 40 0 million year s ago until the close of the Creta ceous P eriod about 65 million year s ago Their sof t tentacled bodies were enclosed in a shell The shell w as di vided into c hamber s that made intricate curved pat terns on the shell Gift from Harrison Cobb Boulder Colorado