I just purchased a bunch of scarab bracelets on ebay and I want to resell some of them with accurate descriptions.
Many of them are hard to distinguish.

Every stone has its distinctive characterstic.

Plastic: Usually it is lighter in weight (amber is an exception)
It is foggier in color. The edges if you see carefully look 'melted' down or 'worn out'. If there is a crack or a chip on the stone then look at the formation of the break. It will be smooth like a plastic break with no specific direction.

Glass: Usually heavier than plastic but lighter than stones. Again this is easier to see in case of loose stones but not for set stones. You may look for 'bubbles' under a loupe in glass. Most stones (Ruby and sapphire) are fairly harder than common glass and you can make out by the chips and breaks on the edges of the facets of the stone. This is subjective to how much the stone has worn out already. If the stone breaks… you will find 'step like' concoidal break formation in it which marks it as real. If it is smooth (like a regular glass object breaks) then it is glass.

Real Stone:
Usually the best of colors are very expensive. If you are buying something that is too good to be true then maybe it is not true. Always try to look in the loupe even if you cannot find anything major inside the stone. Look for bubbles which are a sure shot indicator that the stone is unreal. Besides that… just go by your instincts and try to buy from a trusted source. Or else get an appraisal done or go to another jeweler with your piece who will be more than happy to tell you that his competitor is selling fake stuff

You can write to me if you need any specific stone related query at surbhi.s.gupta@gmail.com

All the best.

What kind of glue do I use to glue jewelry stones to material.?