I went through my collection of rocks that I picked on a beach in Greece nineteen years ago, and found this interesting one. It's very strong blue and looks almost plastic. What could it be?
Definitely not glass... i was thinking maybe blue lace agate? 
If you spill molten glass on gravel or sand you get matrix. 
It's glass as Reiner explained. Yes, almost certainly glass, but just to tweak your interest, here is a somewhat similar example of dark blue Indiana chalcedony with similar markings. I have several others as well. A small piece, when wet, might look very similar to the posted photo. CHEERS.....BOB
