Not sure possible locket

[Amce54]

I'm not sure exactly the use besides jewelry but I really wanna what type of stone this is that is inside the brass piece. Could me glass as it feels like it but it is too shiny and consistent with some type of stones. It is almost 2 centimeter long by 2/10 of a centimeter short. As well 1/10 of a centimeter short of 11/2 centimeter wide. I just noticed under light that it was once gilded in gold. If a home knows the stone of reason for use It would be much appreciated. It also has some type of glue type composite in the inside of the stone.

 

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[Back-of-the-boat]

Might be black onyx

 

[Msbeepbeep]

Looks like onyx.

 

[Amce54]

Is it somewhat rare?

 

[Back-of-the-boat]

No not really