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Calcite is found in nature in optical grade. it also comes in a variety called "Iceland Spar" which is also optical quality and is double refractive.

A softener has nothing to do with grit and sand. it just affects the carbonates in the water. As a matter of fact grit in the water will destroy the resin in your tanks. Most softeners have a filter...

Actually considering the water quality as a factor is in the same realm as changes in the barometric pressure and status of the moon and humidity and temperature conditions affecting the end result...

People have been honing with whatever water they have for centuries with no ill effects on the hones or the razors

That's good enough for me. I use tap water.