Pretty sure its either glass or a man made stone but just take it to a jeweler and let them look at it !! One look and they can tell what it is !! Any kind of a real gemstone would not be mounted in metal like that !! It would be in gold or sterling silver which is always marked !! And they are not drilled for the mounting !!
Yours seems to have had the hole in top made at the same time the stone was !! Like melted and poured into a mold !!
I had several similar stones back in the 60`s which were very pretty but just costume jewelry !!