It might be also striped flint; more or less regular system of concentric dark and pale stripes, resembling rolling waters is typical for that stone. It's very rare, it comes from only one place in Poland.
the best thing at this point is to test and see if it is glass or stone. Touch them to your face. Are they extremely cold? If they are very cold they are stone. If not then they are glass. If you are not sure how cold they should be then compare with a known stone piece or touch a glass to your face and see if they are colder than that. Once you get the sense of the temperature you won't have to compare, you will know!
