Get multiple paperwork proof of it's value and characteristics, and then sell it yourself. If it is worth $18k, you will make the most money by selling it directly, and not to a jewelry store. A consignment option might even work.
I have a HUGE star sapphire (my grandfather was a jeweler from Switzerland). I've never seen a star sapphire in real life, and those I've seen were tiny compared to this. First time I got it appraised at a jewelry store, the guy told me it was worth around $1,000 and he'd buy it if I wanted to sell. Second time when I got it appraised by a jeweler (not at a store), and told him I don't want to sell it, but just know the worth... he told me $4,000+.