As part of my mustachian conversion i had recently taken to selling some old jewellery, and it was an eye opening experience. It is not at all a good store of value and if you have a piece of gold/silver or diamond jewellery expect to get 10% - 20% of retail back as scrap. Small stones many jewellers will not even take as the simply have so many and are able to get them so cheaply.
What the jeweller valued the stone at i would be surprised if you are offered more than 50 for a stone that retails for 250. That being said i would look online as there are some players in this space that will purchase stones. If the stone retails for 500 local jewlers will definitely make an offer of some kind on a stone that size. Just dont expect it to be 500. Likely closer to 100.