Limestone in a shower

    We have green limestone in a shower. Within 6 months, the tiles were
discoloring and the surface looked like it had soap scum/residue on
it. The tiles on the front of the threshold began showing signs of
spalling and developed a scaly finish. The top of the threshold
looked like it had been walked on so much that the surface was worn
away and didn't look like the other tiles at all. The back of the
threshold inside of the shower looks fine after I cleaned it with a
poultice by Miracle. We have soft water which some have said could
cause the spawling problem, but that doesn't explain the front of the
threshold that doesn't get wet like inside of the shower. I have
heard that green limestone or any limestone should not be used in a
shower. We've had several restoration companies try to restore sample
areas, but nothing has worked. Any ideas as to what may be the problem?