how to tell the difference

A simple acid sensitivity test can be performed to determine whether a stone is calcareous or siliceous. You will need about 4 oz. of a 10% solution of muriatic acid and an eye-dropper. Or you can use household vinegar and an eyedropper. Because this test may permanently etch the stone, select an out of the way area(a corner or closet) and several inches away from the mortar joint. 

Apply a few drops of the acid solution to the stone surface on an area about the size of a quarter. If the stone is calcareous, the acid drops will begin to bubble or fizz vigorously. If little or no reaction occurs, the stone can be considered siliceous. Rinse the area thoroughly with clean water and wipe dry. This test may not be effective if surface sealers or liquid polishes have been applied. If an old sealer is present, chip a small piece of stone away and apply the acid solution to the fractured surface. 

CAUTION: Muriatic acid is corrosive and is considered to be a hazardous substance. Proper head and body protection is necessary when acid is used.
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