They were originally coded as a completely different monster, whereas the Gargoyle Pick Elementals were coded as variants of the Earth elemental (and that code was reused for Shadow elementals in the Yomotsu Mines and when they appeared during the candle of Love event as event monsters - even the Blackrock elemental is a peerless-level Valorite elemental - and IDs as a valorite ele to Enemy of one).
The old wild-spawning verites and other free-ranging ore elementals from before AoS were the same way, from what I understand. Earth Elemental Slayers didn't work on DC elementals (And may still not - they were supposed to have fixed this about a year or so ago, but can't remember if they actually did so), requiring the use of an elemental super-slayer. DC elementals can currently be found in Terort Skitas (Sorcerors dungeon) in Ilshenar, The Yomotsu mines, Shame, and a single DC elemental roaming the southern boundary of Compassion Desert (near where the Human to Elf quest Sphynx spawns).
The Dread Spiders of the Treasure maps are the same way. They are literally a different monster than Dread Spiders elsewhere - different loot tables, and they were unaffected when Dreads got that gold-farmer-killer upgrade a couple years back that temporarily made Mephitis spawns unworkable, until they debuffed them and weakened the loot instead (at the same time as the debuff they buffed the champs - but for the duration of the "Super-dreads", it was possible for Dread Spider paragons to spawn at the Ilshenar champs with more HP - and more lethal attacks - than Mephitis herself!). Not even the Devs realized this, when the subject of the Super-dreads came up at Columbus 2007 Town Hall, and I pointed out that the T-maps were unaffected - they quickly looked, and realized what had happened.
BTW, be careful if you get a shadow iron elemental - they are immune to pets, summons and most spells (I think that only Earthquake works). However, wild creatures can and do hurt them (like pixies, Ki-rin, Unicorns and Meer - as you can tell, I mine in Ilshenar a lot) and I THINK that they can be provoked into fights that will kill them. Coppers have 50% damage reflect (melee and spell, I believe, but not archery), Bronzes have a ranged gas attack, and Verites can break anything equipped that isn't blessed or insured (and I think cursed items might be immune as well). Valorites have the abilities of ALL the ore elementals combined (immunities, damage reflect, area attack and item destruction).
