Heat-treated quartz crystals, a beautiful specimen showing the crystal structure of this small specimen. This piece is about 1 1/2" long. Sometimes crystals are treated with heat and also a chemical wash to bring out different qualities needed for manufacturing or gemology.
One description of the heat treatment for crystal specimens:
Heat treatment or 'annealing' is done using a combination of chemicals (beryllium, borax, lead, tantalum) and heat to permanently alter a stone's color. Heating or "cooking" the stone is done at temperatures ranging from 450º to 1850º Celsius for a period of 2 to 12 hours or more. One telltale sign of heat-treatment is the presence of small fractures or "decrepitation feather" within the stone that appear around natural mineral inclusions."