Well, not really 'plastic', but Cast Polymer (also called faux marble - the stuff many bathroom countertops are made of).
This stuff can be purchased as sheet goods (like for a shower/tub surround). During the install process, I managed to crack one of the sheets. At $200-$300 per sheet, I don't want to replace the whole thing, I'd just like to cover/seal the crack in a way that it blends - I know it won't be perfect.
The 2 pieces have been fastened to the wall, lining-up the crack as good as possible. I've thought about using a 2-part epoxy, then letting it harden and polish it out. I'd hate to have to use something like a router to cut a groove, seeing that the crack isn't a perfectly straight-line, and is approx 24" long.