One wiseacre assumed we meant any package leaving our shipping room that once had a glass object in it. That is a malicious rumor. . . Ground glass is thin plate glass, the surface of which has been physically ground or acid-etched to create a grainy, grayish, opaque surface. Use it to produce an image from a lens or lens system. Even a simple magnifier, held between the ground glass and, say, a window, will produce an image of the window scene on the ground glass. Ground glass has another magical property: it is opaque to any object viewed at a distance, but not if that object is in direct contact with the glass surface. A necessary element of your optical education. 3-1/2" x 4-3/8" x 1/16" thick.