There are a couple of things that would be helpful. Since you didn't find the item it was mounted in, it is hard to estimate the type/value of the stone. If it had been found in an 18kt mounting, we would be pretty certain that it was worth being put in that nice of a piece of jewelry. If it is an Emerald, they almost always include some types of flaws or inclusions. Just the way the stone is. If it's Jade, it most likely won't be transparent. If it's Malachite, it will show some banding. So a picture with some light passing though it will tell a lot about the stone. If it is a stone. It could be a cast piece of resin. The best way to tell this is to take a sewing needle and heat it to red hot at the tip. Try to push it in a place on the back of the stone. If it goes in or makes an indentation. It is not a stone.
See if you can get a picture that shows the inside of the stone. With green stones, that is a key factor in trying to help ID it.
As far as taking it to a jewelry store, there are very few real jewelers left out there. The store owner may have taken a course of some kind on diamonds but that is about as far as it goes. Try to find a goldsmith. They usually have a better knowledge of stones and such. Whatever either of them say, don't sell it. Get more information.
Daryl