My un-educated theory? Limestone varies in hardness based on the quarry, hardness of the particular seam being mined, ect. Leuters limestone (which is what my coping is) is from a quarry in Leuters Texas that is known for limestone that is particularly hard. However, limestone is a natural product and therefore somewhat un-predictable. The coping stones around my spa are larger stones than the rest of the pool. My guess is that the larger (spa) stones were mined from a different seam than the rest of the pool. The pool stones were probably mined from a seam that had a stretch of soft stone in it. I probably got that batch of stone that is too soft to be used as coping. For that matter, who knows if the stuff really came from Leuters. The name has become analagous with hard stone and a stone vendor can call any stone Leuters and who would know the difference? Just my therory, but i know that a lot of limestone, for instance the flagstone on my deck, is soft and flakes, and would not be suitable for use as pool coping. So, I am going to have it replaced and will apply the best sealer i can find, but based on what i have learned here, the salt water generator is irelevant as to whether it will detoriorate or not.