In order to find them, go up the rope bridges southeast of the Frontier Village entrance. There is a mountain blacked out on the map immediately west of where Clear Waterfall is labeled on the map. Stand on the rope platform directly south of this mountain and face south (toward the open field below). A Pterix will fly by in front of the party, circling the mountain. As it drops HP Steal crystals even in silver chests, it provides an easy target to farm these gems using Riki's skill links to increase silver/gold chest drop rates for all three party members (adding up to over a 50% chance of getting a silver drop).