Jak 3

Metal Head Skull Gems can be obtained by defeating enemies. There are a couple that will spawn in crates, but most of the ones you earn will be from killing enemies. There are a few areas that you can simply farm monster that continue to respawn and drop a Gem every time they are defeated. There is a great area just after the Bot Boss (right after the Eco Mine) where a group of small, easy to defeat monsters spawns continuously, and around half of them drop Skull Gems each time you defeat them. Personally, I stayed there for around ten minutes and farmed a couple hundred Gems, just to get it out of the way. This trophy is cumulative, so feel free to spend as many Gems as you want.