The skull gem, also known as the gold gem; in the case of metal heads specifically the metal head gem[a] or metal head skull gem; and in the case of metal bugs the gold bug gem or metal bug gem; is a collectible in Daxter, Jak II, and Jak 3. It is a bright yellow gem embedded in the head, chest, or back of a metal head, including metal bugs (from hereafter the term "metal head" will be used to include metal bugs).
The purpose a skull gem serves to a metal head remains unknown, though it does seem to function as an organ, as it usually connects to a tube that leads to elsewhere on the metal head's body (excluding metal bugs and some smaller metal heads like the hopper or stinger).
Furthermore, most metal heads will eject their skull gem upon death, which could mean the gem acts as a source of life or energy. This also makes it a valuable collectible, as evidence for the demise of the respective amount of metal heads.
As such, in Daxter, skull gems were the objective of four missions, "Bring back 25 gems to the Concierge", "Collect 30 gems from the tanker", "Collect 40 gems (distillery)", and "Collect 40 gems (fish cannery)". In those missions, the assigners required the respective amounts as proof of completion of the extermination job.
In Jak II, the Water Slums oracle would give Jak dark powers in exchange for a set amount gems, which proved Jak capable of defeating the "metal scourge". In Jak 3, Jak instead used skull gems to activate side mission computers. Unlike in Daxter and Jak II, Jak could collect skull gems from more than just a killed metal head, but also from vases and crates.
According to the sound produced when it falls, the skull gem is lightweight and made of something of a shatterproof glass. It is ovoid in shape, glowing with what seems to be a yellow liquid energy, magnified and refracted by its presumably glass-like shell. When collected, a brief yellow flash follows its disappearance from the ground.
Metal heads killed by AI (such as Ashelin, Sig, or other enemies), or by unfortunate circumstance (falling), still produce a skull gem, if applicable (i.e., the player does not have to kill it in order to receive the gem of a dead metal head). For metal heads that either fly or swim (wasp, saw fish, some metal jackets, etc.) and whose skull gems would thus become inaccessible upon being defeated, the skull gem will fly directly towards Jak without him having to retrieve it.
For metal heads that were killed by being knocked out of bounds (from large heights, into water or dark eco, etc.) the skull gem will usually launch back on to a steady platform where it can be collected. However, the skull gem is especially prone to glitching (see below), and does not always come back to a sufficient place for collection.
Collecting skull gems is also related to trophies titled "Head Over Heels", "Head Cheese", and "Head Master" in Jak II, as well as "Oh My Head", "My Head Hurts", and "Head Over Here" in Jak 3. Collecting 50, 250, and 510 skull gems unlocks a bronze, silver, and gold trophy in the former, and 25, 125, 250 unlocks equivalent trophies in the latter, respectively.
In Jak II and Jak 3, during the progress of a game, only one skull gem is produced from each metal head spawn, meaning that if the metal head ejects its skull gem and the player collects it, it will not retain the gem by the time it respawns, which is usually either after a mission is completed, or when Jak is killed. In some missions, mass amounts of metal heads are encountered, and it is often the case that a metal head will not contain a skull gem regardless of whether or not Jak has died. This is likely because the same enemy seed has respawned after being destroyed, losing its skull gem. In this case, the skull gem evidently serves as a respawn indicator. This, however, is not the case after the game is completed, for skull gems are found in an infinite quantity, as the number in which they are collected does not matter.
To add, during swarms of a finite amount of metal heads (like "Find pumping station patrol" and "Intercept tanker" from Jak II), if one avoids collecting the skull gem ejected by a metal head, by the time that same metal head respawns it will have yet another gem, as the game did not register that the original gem was collected, allowing the player to effectively cheat the system and collect multiple gems for a single metal head.
However, in Daxter, skull gems are deducted from the player's count when Daxter dies, depending on mission progress. The skull gems will only be saved if you reach a check point after collecting them. Daxter is also the only game where you can check how many skull gems can be collected from a level and where skull gems are not found in infinite quantity after completing the game.
Skull gems are particularly prone to glitching out of the game's environment. They may be rendered uncollectable by falling in a body of water, falling straight through the ground, sliding towards an unreachable location, or by just disappearing. The only way to get it is to revisit the location after respawning, because even if the skull gem is ejected, the metal head will retain it so long as Jak never collected it.
- ↑ Curiously, the term "metal head gem" is used in this source to refer to metal bugs.