It could be one of two things;
B. Tartaric acid crystals: a harmless malady that forms after the bottle has not been properly stored. or, C. Black tannic deposits: a deposit of tannin and other colored materials that form after years of aging. Affects mostly red wines, and is not a sign of bad wine.
There are other maladies that can affect wine’s quality, but they are very rare.
Posted in: Wine