Calcium Scale
Calcium scale forms on pool surfaces due to very hard and out-of-balance pool water. Calcium scale is usually uniform and covers both plaster and pebbles in a pool. Calcium scale usually feels rough, like sandpaper, on the pool surfaces.
Calcium scale can occur on plaster, vinyl liner, or fiberglass pool surfaces.
Calcium scale is normally from calcium carbonate but can be from calcium sulfate (CaSO4). Aqua Magazine:The Mystery of the Pointed Crystals describes a calcium sulfate scale situation. Orenda Understanding Calcium Sulfate Scale Crystals further describes calcium sulfate scaling and solutions for it.
Calcium phosphate can also form scale and we rarely see that in pools.
Calcium scale is different then waterline scale on tiles, evaporative scale on surfaces outside of the water, efflorescence, or calcium nodules.