The Obsidian Rose is an accessory that reduces lava's base damage from 80 to 35 / 50. It has a 2*1/50 (2%) chance of being dropped by Fire Imps, an enemy found in The Underworld.
More effective and easily accessible means of surviving contact with lava, such as Obsidian Skin Potions and Lava Charms, become available during or even before the stage of progression where a player could reasonably begin looking for an Obsidian Rose. Combined with how rarely it appears, and the fact that all its alternatives prevent lava damage outright, an Obsidian Rose has usually outlived its usefulness before it is even found.
If the player has an Obsidian Rose and at least 97 / 66 defense, they will only take one damage from lava. The On Fire!debuff from lava will still deal full damage unless the player is also submerged in honey. This can be exploited to create an invincibility machine.
The damage reduction from the Obsidian Rose or accessories crafted from it do not stack; wearing two or more does not reduce the base damage of lava below 35 / 50.
The Obsidian Rose can be dropped by Fire Imps spawned from the Imp Statue. [1] (Albeit at a reduced drop rate of 0.1% rather than the usual 2%)