Will a rock that raises the ph ever stop raising the ph?
I saw a rock I absolutely love at my lfs and was told it will raise the ph so I didn't buy it. It's got some nice quarter to half dollar sized holes in it that my ghosties would love.
It will most certainly NOT keep raising pH ad infinitum. It will level off at a certain point, well within what is safe for the majority of aquarium fish.
Different forms of limestone require different pHs to make them dissolve. If you keep your pH above 8.5 no form of limestone I'm aware of will dissolve. Aragonite will dissolve at just below that, dolomite needs a pH in the mid 7s, and calcite requires even lower pH.