Well first, you need to set what the maximum amount the richest person that could be interested in it, is willing to spend on it (even if it's lower than what it's worth, if no one has enough funds they won't get full value). They need to feel out the market in the hopes of finding a potential buyer. This would indirectly cause information to leak out to people who keep an ear for this sort of stuff (hush money doesn't guarantee anything) and this will attract criminals. Attempts to steal both the gem and the payment will be made until the exchange is made. They can also give it as a gift to an important person or an organization in exchange for political favours (or maybe magical favours)