Yeah, I assumed it would be glass since they're set in silver rather than gold, but I've seen some rings where a small ruby or sapphire were set in a Sterling ring. For the price, I figure it'll help me avoid throwing out a real stone at some point.
!BBH
!PIZZA
!ALIVE
Like you said, you can never know until you really look into it.
Some of the most valuable (expensive) stuff from the Victorian age is costume jewellery.
And Cubic Zirconium, almost exactly like diamonds, except it doesn't dissipate heat like a real rock.
So, keep testing.