Golem Core

A Golem Core is a piece of forgeable material in Golden Sun: The Lost Age. It is described as "An Earth Elemental's heart". It is found in various areas after the Great Western Sea is accessed, such as in Loho, and it can also be dropped rarely from a Bombander in Anemos Sanctum (fell it with Mercury Djinn to increase drop rate by four times). Being an Artifact, it can be sold and later repurchased. It can be bought for 1500 coins and sold for 1125 coins. It can be forged into the following quite powerful equipment:

Chronos Mail: Armor class: Defense +47: Maximum HP +20. It is bought for 13100 coins and sold for 9825 coins. The chance of it being forged is 30%. Gaia's Axe: Axe class: Attack +163: Unleashes Mother Earth, which converts to Venus damage, adds 56 attack points, and has a chance to put the target to sleep. It is bought for 16400 coins and sold for 12300 coins. The chance of it being forged is 15%. Huge Sword: Long Sword class: Attack +155: Unleashes Heavy Divide, which converts to Venus damage, adds 60 attack points, and cuts enemy defense by 25%. It is bought for 14000 coins and sold for 10500 coins. The chance of it being forged is 15%. Titan Gloves: Glove class: Defense +43: Maximum HP +30. It is bought for 8100 coins and sold for 6075 coins. The chance of it being forged is 25%. Tungsten Mace: Mace class: Attack +159: Unleashes Hammersphere, which converts to Venus damage, adds 51 attack points, and may lower the target’s defense down to 50%. It is bought for 14700 coins and sold for 11025 coins. The chance of it being forged is 15%.

Analysis
Golem Core weapons are strong (perhaps equally so), though ideally you would have even better weapons than those by the endgame. The Chronos Mail and Titan Gloves are pretty often worn for strong defense ratings and good HP increases.