The Firestone was a warlock-only temporary weapon enchants akin to wizard oils and created by the [Create Firestone] spell. A Firestone increased all spell direct damage done by 1% and increased spell critical strike by 7/14/21/28/35/42/49. They were bind on pickup, but unlike other Conjured Items, Firestones and Spellstones did not vanish after logged out for any amount of time (Much like a Wizard Oil created by Enchanting).
Firestones and Spellstones were the preferred temporary weapon enchants for Warlocks. Firestones were preferred for Warlocks that favor direct damage spells (Destruction and Demonology warlocks), while Spellstones were preferred for Affliction Warlocks, who favor damage over time spells.
The original Firestone was an off-handslotconjured item which was later moved to the wandslot (technically a relic), created by the warlock Create Firestone spell, allowing warlocks to enhance their main handweapon to do additional firedamage and gain a fire spell damage bonus buff for as long as the Firestone existed. When active, a Firestone stacked with weapon enchantments, and placed a small fireball that circled the Warlock's weapon. Both the Firestone and Spellstone were widely unused for multiple reasons. They did not provide comparable stats to wands and, like other conjured items, would disappear if logged out for more than 15 minutes.
Types[]
Types of firestones
Type
Item level
Damage dealt by direct spells
Spell critical strike rating
Lesser Firestone
28
+1%
+7
Firestone
36
+1%
+14
Greater Firestone
46
+1%
+21
Major Firestone
56
+1%
+28
Master Firestone
66
+1%
+35
Fel Firestone
74
+1%
+42
Grand Firestone
80
+1%
+49
Patch changes[]
Patch 4.0.1 (2010-10-12): Create Firestone removed from game.
[Create Firestone] spell. A Firestone increased all spell direct damage done by 1% and increased spell critical strike by 7/14/21/28/35/42/49. They were bind on pickup, but unlike other Conjured Items, Firestones and Spellstones did not vanish after logged out for any amount of time (Much like a Wizard Oil created by Enchanting).
Patch 4.0.1 (2010-10-12): Create Firestone removed from game. 
Wowhead Firestone info.
