Obsidian is a volcanic glass that was used by prehistoric people as a raw material in the manufacture of stone tools such as projectile points, knives, or other cutting tools through knapping, or breaking off pieces in a controlled manner.
Obsidian obeys the property of mineral hydration, and absorbs water, when exposed to air, at well defined rate.
The rate of penetration of water is dependent upon several factors, primarily the chemical composition of the glass and the temperature at which the hydration occurred.
Discussions are given of techniques for measuring the hydration thickness, measurement (or estimates) of ambient hydration temperature, chemical composition of the obsidian, and the conversion of hydration thickness to dating the time of manufacture of the artifact.
When this hydrated layer or rind reaches a thickness of about 0.5 microns, it becomes recognizable as a birefringent rim when observed as a thin section under a microscope.
Hydration rims formed on artifacts can vary in width from less than one micron for items from the early historic period to nearly 30 microns for early sites in Africa (Michels et al. Formation of the hydration rim is affected not only by time but also by several other variables.
In its natural state, it has a thick rind formed by the diffusion of the water into the atmosphere when it first cooled--the technical term is "hydrated layer".
Although the depth of penetration can be measured by various methods, it is generally determined by microscopic examination on thin sections of the artifact cut normal to the surface.
Obsidian outcrops all over the world, and was preferentially used by stone tool makers because it is very easy to work with, it is very sharp when broken, and it comes in a variety of vivid colors, black, orange, red, green and clear.
Obsidian contains water trapped in it during its formation.
C dating, thermoluminescence is related to radioactive decay.
Thermoluminescence is produced by radioactive decay particles (electrons), trapped in mineral grains.