Is it a part of old glass coaster?

[kuna12]

I found this piece of red sea-glass, and thought it might be a bottom of an old bottle. But looking at the reverse side, it's pretty clear that it's not any kind of bottom, it was always this flat - a coaster, perhaps? Can't find anything like this in Google Images (it usually works).

It's broken, but I think the embossing reads: DIAMOND, Diamond Heat Resistant Glass (inside the diamond), and word FOR. But for what? Please, help - I'm very curious to find out. Thank you!

 

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[No gold in NY]

Interesting piece of glass. Edges look like it was eroded in a stream. Keep looking, you may come up with an answer.

 

[kuna12]

In the ocean. The closest thing I found - Kopp heat resisting (not resistant) glass lens as automobile light cover. But it's convex, the glass I found is flat.