I got this as part of an eBay lot of drawing/drafting equipment. It looks like a cutting board but it has a hard insert (feels like a very hard plastic).
I think that's epoxy resin with some glitter in it. So very hard plastic is spot on. My guess is that this is a cutting board with a "decorative" resin poured into a routed dado. The previous owner may have used it as a makeshift drafting table or drawing surface.