i used to work with amber a lot when i first started jewellery making - i used to buy it raw and then polish it up.
you could always try the static test if there is enough of a dome shape to the ring?
rub it on something like carpet or a woollen sweater and then see if it picks up tiny pieces of paper, glitter or fluff.
but this test will only tell you if it is amber and not true amber in the sense of it being a lump that has been shaped and polished, and not re-constituted amber.
this is where they melt it and then pour it into moulds, and you can usually tell if there are lots of spangles inside it - like little round sunburst shapes.
here is a pic of what i mean... http://www.mineralminers.com/images/...ry/ambj126.jpg
it is still amber, but not natural occuring amber.
hope this helps you!