Certain glassy appearing minerals like obsidian are really undercooled substances, natural glasses, which have cooled without crystallizing. In the course of time they begin to crystallize. It is not unusual to find in nature numerous samples of these minerals existing in all stages: the metastable, partially 
Fig. 6. Crystals of Needle-like or Monoclinic Sulphur. When first formed they are bright yellow. After standing for a few hours they change to a dull yellow color and become very brittle. A microscopic examination of a fragment shows that it is now made up of minute rhombic crystals (Fig. 7). crystallized, and the stable or completely crystallized mineral.
The rate of the growth of crystals is a specific property depending on the substance used. Much information has been obtained on this subject by studying the rate at which crystals are deposited from undercooled liquids. By filling a narrow glass tube with the desired liquid, as shown in figure 2. inoculating at one end and measuring the time necessary for the crystal surface to travel the length of the tube, the rate of growth of a crystal can be measured. Undercooled phosphorus crystallizes 200 feet a minute, water at two degrees below zero at the rate of 8 inches a minute, 
Crystals of Rhombic Sulphur. while one thirty-second of an inch a minute is the velocity of crystallization of betol. In nature crystals often grow much more slowly than this. The examples cited above deal with undercooled liquids, but the phenomenon of undercooling is by no means restricted to this class of substances.
When molten sulphur is allowed to cool slowly, long lustrous needle shaped (monoclinic) crystals separate from the liquid. On standing for a few days the appearance of the crystals changes; they lose their luster, and examination with the microscope shows that their crystalline form is no longer needle-like, but consists of so-called rhombic figures. The temperature at which the transition from needles to the rhombic form takes place is 96°.
