If you mean the purple emerald, it's on one of the speakers upstairs.
Either shoot it with the shotgun (behind the counter) or use the hoverboard near the couch upstairs to get up and grab it. You do this by placing it under you and grabbing it.
Okay it's been a while but I saw in sea magic, (made by epicpikaguy) he implemented buttons to use arrows and such, and if it's in your power, you should do this to your new games, it should make it compatible even to people that don't have PCs, which would be useful.