Can you show the scale?
It it clear or white?
Is it a shaped crystal?
Is there any color like blue, turquoise, green or black?
Calcium sulfate crystals tend to be clear with a swallowtail structure.
In the below case, they used boiling acid, but that is very dangerous.
@onBalance was involved in this case.
Seemingly overnight, this pool was covered in sharp crystals. Where did they come from?
www.aquamagazine.com
"I called up my partner, Kim Skinner, and told him the facts of the case.
After some consideration, he suggested that the deposits might be another form of calcium than the normal calcium carbonate — such as calcium sulfate.
Calcium sulfate, CaSO4 (commonly called gypsum) can form crystals as evaporates in pools, and they are distinctive from the common calcium carbonate precipitates you often see in several key ways: First, they are softer (only 2 on the Mohs scale as opposed to calcium carbonate, which is a 3).
Secondly, they are soluble in acid only if that acid is at boiling temperatures.
And finally, they form what is called a “swallow-tail” crystalline structure.
Since the deposition process is almost entirely an evaporative one, most if not all of the crystals will be at the tile or water surface line."