Like... [Digs around in his bag for his own stone (http://bulbapedia.bulbagarden.net/wiki/Evolutionary_stone#Fire_Stone).] Like this.

Ven had one in blue, and Eevee became a water-type. This one's yellow, so you'd think it would make Eevee an electric-type, but I can see some red markings here that look like fire, so maybe it's one or the other.