Primordial Earth Heart
This crystal-pierced rock is shaped like a heart. It gently pulses while held and is always slightly warmer than body temperature. It is said that the first earth elemental was once a human that joined with such a heart.
As part of attuning to the heart, you must also hold it to your chest. Once you've attuned to it, the heart magically sinks into your chest, with its crystals just barely visible above the surface. While attuned to the heart, your skin becomes hard like stone, granting you the effects of the stoneskin spell. In addition, while you are not wearing any armor, your Armor Class equals 14 + your Dexterity modifier. You can use a shield and still gain this benefit.
Spells. While attuned to the heart, you can use an action to cast one of the following spells from it (save DC 17): entangle, spike growth, or wall of stone. Each spell manifests as a series of jagged crystals. When a creature other than you enters the area of one of these spells (or within 5 feet of a wall of stone) for the first time on a turn or starts its turn there, that creature immediately takes 1d4 piercing damage from the jagged crystals. You are immune to the effects of the entangle and spike growth spells when cast in this way. Once the heart has been used to cast a spell, it can’t be used to cast that spell again until the next dawn.
Curse. This heart is cursed, and attuning to it extends that curse to you. While cursed, the stone and crystals spread across your body, gradually replacing your flesh with pure rock and crystal over the course of 30 days. After 30 days, you no longer need to breathe, you gain a burrowing speed equal to half your walking speed, and you have tremorsense out to a range of 10 feet. Your weight is also tripled; your swimming speed is reduced to 0 if you have one, and you sink to the bottom of any liquid at a rate of 60 feet per round, at which point you can walk as normal. Only a wish spell can end your attunement to the heart and remove the curse, at which point it erupts from your body, landing in a space within 5 feet of you and returning your flesh to normal.
You have to understand: he's a myth. Never had proof, you see, and it's not odd to think ye see things in pitch dark. So when the *blast ooze* went off early and turned the supports t' splinters, whole crew knew we were dead. Roof of the cave knew it too—a whole lotta weight groanin' with nowhere to go but down. Then the floor cracked open an' jagged gems punched through, all the way t' the ceiling. No shame to say we took that miracle and ran, most of us just with cuts and bruises. I was last out, and when I looked back… I won't forget that diamond grin shining at the tunnel's end, before the crystals vanished and it all fell apart. Nobody else saw, but… I know it wasn't just fantasy. He's still down there, somewhere. I know it. And while he is, I'll do what good I can to pay it forward. —A miner's report of an encounter with Terr Gallum, excerpt from *Exiles of the Planes*