QUESTIONI have a marble shower in which the wall marble tile is 12"x24" x3/8" cararra marble.
We are getting many "smaller than" hairline cracks in the marble in a half a dozen places. You can barely even feel them.
It's a large shower. The walls are 6' wide and 9' tall.
ANSWERANSWER - Generally speaking cracks are either reflective cracking where the underlying substrate has a crack or separation and it telegraphs through the marble or there is structural cracking where the structure has excessive deflection or other structural issues. Or it is a compressive crack where the stone is not fully supported with the thin-set adhesive that leaves voids and there is a load applied to the stone causing the crack where it isn't supported.
Since you said it is smaller than a hairline crack and you can barely feel it, it leads me to think that you might have what we call indent fractures. Typically this is caused by excessive shrinkage in the adhesive, if it is applied too thick, or within the substrate if it develops shrinkage cracks for one reason or another.
Initially indent fractures don't actual crack the surface. It is a very slight depression. If you put a straight edge over it and shine a flashlight behind the straight edge you will see a small gap at the indent. Typically you can only see these indent fractures from an angle when light shines on it, otherwise it isn't notice able.
Indent fractures can develop into actual cracks if it is subjected to enough stress since it is a weak point, but often they don't crack. If it is an actual crack then you have to remove the tile and look for the evidence to explain what caused the crack, and the determine how best to remediate the problem.