As of Patch 3.0.2, Firestones and Spellstones have replaced [Superior Wizard Oil] (since Wizard Oil can not be applied to weapons with an Item Level higher than 165) as the preferred temporary weapon enchants for warlocks. Firestones are best for deep destruction specced warlocks, whereas spellstone is better for demonology (with the [Master Conjuror] talent) and affliction.
Patch 4.0.1 (2010-10-12): Create Spellstone removed from game
Patch 3.0.2 (2008-10-14): Redesigned. Spellstones are now considered a weapon enchant, and now have 5 charges. No longer a Conjured Item.
Patch 2.0.1 (2006-12-05): Spellstones no longer absorb magic damage, and grant spell critical strike rating instead of 1% spell critical chance. Now equipped in the wand slot instead of the off-hand slot. Spellstones now have a 3-minute cooldown, but are no longer consumed when used.
Patch 1.7.0 (2005-09-13): When a Spellstone is equipped, it should no longer interrupt tradeskill creation.
Patch 1.3.0 (2005-03-07): Spellstones now increase your spell critical strike chance by 1%.