The Iron Golem is unique in that it requires an item to be created. This can be anything from a cracked Short Sword to a Breath of the Dying runeword. The item has to be of metal though, and it must be possible to equip. Once created, the item is lost forever, so think twice before using a valuable item.
Raising an Iron Golem from an Ethereal item makes the Iron Golem translucent.
While many mages spend their lives devoted to studying the transmutation of base metals into gold, the Necromancers have always had a somewhat different approach to transforming metals. Through complicated arcane rituals and great mental concentration, a Necromancer can summon forth a Golem from common base metals. The construct takes on the properties of the original source material, including any magical effects or other properties the original metal might have possessed.
↑Diablo II Manual, page 77. Blizzard Entertainment, 2000.