interaction of grape juice or wine with marble I have a marble cup which I drink wine from. If I leave a little bit of wine in the cup and wait maybe 30 minutes, the wine becomes discolored and appears black (photos attached). My conjecture is that this is the acid from the wine interacting with the calcium carbonate of the marble to produce calcium choloride? If so, is it toxic to continue drinking wine from the cup? Thanks.
CaCl2 and magnesium are not toxic at these levels. In fact NIH suggests around 1 g calcium per day: https://medlineplus.gov/magazine/issues/winter11/articles/winter11pg12.html

Marble itself is predominantly calcite or dolomite, that is carbonates of calcium and magnesium. Marble also contains other elements. We can't say if these other chemicals are safe just by a photo because composition of marble varies. 

When you bought this cup - did they say it is safe to drink from it? Supplier should know if the material is safe or not. If you didn't get assurance that it is safe then you should assume it is not.