Featured Pierced Silver Brooch- Old or new? Stone or glass?

Good morning Ladies,
I'm trying to figure age and stone type on this. It is only 2" x .75" so it's a tiny and delicate thing. The 70-something owner says it belonged to her grandmother. I haven't tested the metal yet (too nervous). No marks.
The roll-over clasp dates it not much earlier than 1960's, correct?
And how on earth do I test glass versus amethyst? It is faceted on both sides.
Thank you all!

 

Sweet! I suspect costume because I can see where the plating has worn on the edge and brassy color metal is showing through. Also, looks like nibbles on the stone, so maybe glass. But test it for metal content, if it is confirmed costume then you can be pretty sure the stone is glass.

 

1920s North Attleboro, Massachusetts costume is my guess. The filigree was hot in the early to mid-1920s and a lot of it came out of Providence and N. Attleboro. They're on top of each other geographically. It was done in costume and precious metal both, but I suspect costume in this case.

 

Really, that old? And specific region? I need to quit learning things on the Internet. It does look rather Gatesbyesque, doesn't it? That would fit in with the timeline better. Thank you!

 

Thank you! I suspect it's plated too, but I haven't cleaned it yet either. It's not what i would call high-quality workmanship, the casting, altho minute is rather crude around the clasps. Is amethyst un-nibbleable?

 

Glass should be more fragile than amethyst, but if knocked around I think amethyst could also be damaged.

 

I'd lay odds it's glass in this case.

 

Thank you, Ladies!

 

This is the older model roll-over clasp, used in the early 20th century.

 

It's very pretty... I like amethyst and silver (or silver-colored) jewelry.