When a surface of freshly-flaked obsidian is exposed to the air, water is slowly absorbed into the surface of the artifact, leaving a narrow hydration rim that is visible under a microscope. The thickness of the rim depends, in part, on the age of the artifact, and provides researchers with the opportunity to directly determine the approximate age of its manufacture.
A thin slice (1-1.5 mm wide) of obsidian has been removed from the lateral edge of the projectile point above as part of the process of measuring the hydration rim on the artifact.
As is visible in the image above, the process of slide preparation is slightly destructive. A thin section of obsidian is first carefully removed from the edge of an artifact. The thin section is then mounted on a glass slide, ground to a thickness of about 30-50 microns (inverted half-way), and is examined with a petrographic microscope. The hydration rim that has formed on the surface of the artifact can then be accurately measured to a precision of approximately +/- 0.2 microns.
Read more about obsidian hydration.