Is it spherical or round but somewhat flat? Is it hard or somewhat pliable or soft as in squishy, as in can it be malformed by being squeezed or dented with a fingernail? If you drop it on the table by say 10 inches by how much does it rebound or bounce?
It does not look like a naturally occurring mineral specimen. If it is pliable, it may be an eyeball lens from a fish. Such are round, fairly hard but not glassy or mineral like, and occur in a variety of colors and opacity. They also survive being "dried out" without much change. Here is a picture of a fish eye lens for comparison.