Portal Stones allow you to transport between two locations. You can find out which gem is a portal stone by dropping all of them, but remember that they have only a certain number of charges.
Portal stones come in two colors (blue and orange) and create a portal of the appropriate color. Portals connect to portals of the opposite color, so entering a blue portal will cause you to exit from an orange one, if any exist. There may be multiple portals of the destination color, in which case one will be choosen randomly. If no destination portals exist, players will simply walk past the portal.
Portal stones are a reference to the Portal video game series.