Quasielemental Plane of Mineral
From glimmering specks in the sand to vast chasms of gypsum, crystals saturate the ground at all scales. From the vibrant reds of the ruby to the deep purples of the amethyst, they span the entire spectrum of color. It is through the crystal that Earth best expresses those cornerstones of life — growth and variation — and nowhere is that more obvious than the Quasielemental Plane of Mineral. Here, the power of the Positive Energy Plane saturates the matter of Earth, bringing about jewels, gems, and crystals of all conceivable varieties, defended from plunder only by those local beings that would not see their homeland scoured of its beauty merely to line the vaults of the avaricious or fill the stomachs of the voracious.
Much as the plane of Earth, Mineral exists as an endless mass of solid material broken only by the occasional elemental pocket; the key difference is that here, that material consists of hard-edged minerals bound together by metallic ores, in as many combinations as can be imagined. Every sort of crystal and every sort of ore exist in Mineral. And not much else, of course; no light, nearly no air, only the barest of heat. Surviving on Mineral is only slightly easier than on Earth; elemental pockets of air and natural chasms seem to be slightly more common here, though perhaps it's simply due to the paucity of travel to Earth itself in comparison. Sound also seems to travel well through the crystals of Mineral, though it's difficult to pinpoint; here, you can hear something hundreds of feet away as though it was right beside you, if not in which direction.
The great natural worth of the plane often known as the Treasure Trove of the Multiverse leaves many to wonder how gold, silver, platinum, or jewels still hold any worth or rarity at all in the greater planes if one can simply gather all they'd want from here. The answer is in the plane's quasielementals, a kind long-since honed into a great, planewide military force devoted solely to defending it from the predations of outside beings. Were they to set their sites beyond, much of the Inner Planes would be at risk, but thankfully they keep their focus inwards, ensuring that only the smallest of ongoing mining operations can proceed. Organized under the rule of Crystalle, greatest of their kind and claimant to the title of Archomental, they claim the entirety of Mineral as their domain; a claim that has often led to discontent from other local peoples such as the tsnng, most recently expressing itself in the Great Crystalline Wars of some centuries hence. Only the dao have managed to forge an alliance with the mineral quasielementals sufficient to allow for regular mining operations, and even they keep their efforts to a relative minimum purely to ensure that alliance persists.
The plane itself has its own means of defense, of course. As on Earth, there is no light and no air, serving as a first barrier to mortals that would seek the plane out. Further, digging tunnels is much more difficult of a task here than on Earth, due to the natural strength of the plane's material. And any tunnels that are dug within the plane are temporary at best, eventually pressing closed by the sheer pressure of the endless matter. Even while open, a tunnel in Mineral can itself be a danger, as the crystals that line its walls are often razor-sharp, slicing up the unwary that brush too close or that wear boots not quite up to the task.  And of course, the greatest natural hazard on Mineral is "fossilization", the tendency for any living material to eventually transmute to pure Mineral itself. Without protection, any mortal being that spends more than a few days upon the plane is almost guaranteed to succumb to this fate.
So removed from the element of Air, spells associated with such are much less effective here; similarly, the lack of atmosphere reduces the effectiveness of any fire-based spells. However, the nature of the plane enhanced the strength of any spells associated with Earth. Spells with the "air" or "fire" descriptors are impeded, while spells with the "earth" descriptor are heightened by 1 spell level.
- Boundary Regions
- Treat the hardness and hit points of the plane as though it were mithral. Every segment of wall "heals" 1 hit point every minute; this can be reduced through various means of reinforcement, though never stopped entirely. This does not apply to natural elemental pockets or the edges of border areas, only to construction. Any contact with walls deals 1d4 damage as though from a cold iron and silver weapon with regards to DR; in tight tunnels, this is inflicted for every five feet of movement.
- Any organic creatures or objects in Mineral must make a Fortitude save once per day against DC 15. Once three saving throws (not necessarily consecutive) have been failed, the subject will become "fossilized" into a crystalline formation; treat them as though they were victims of flesh to stone with regards to reversing the effect. This counter resets upon leaving the plane. Any inorganic material held by or part of a creature or object is not affected by fossilization, and any immunity from or resistance to petrification of any degree negates this effect.
- The Inner Planes, pgs.13,96-99
- Planescape Campaign Setting - DM's Guide, pgs.12,36
- Planescape Monstrous Compendium Appendix III, pgs.11,84