It has nothing to do with the quality of the wine - read on:
From Epicurus.com:
Tartaric acid
One of a number of naturally occurring grape acids which contribute to the acidity of a wine. Other important acids include acetic, malic, lactic, citric and carbonic acid.
Tartrate crystals
During fermentation tartaric acid may be converted into potassium hydrogen tartrate, formed through its reaction with potassium. This compound may crystallise, when conditions are cold, to form small crystals in the wine. These are small, clear or white crystals. Some winemakers wish to prevent their formation and thus perform cold stabilisation. The crystals themselves are harmless and natural so the decision is a matter of aesthetics. See our advisory page on wine faults for more information.
Tartrates
Beautiful, natural and totally harmless crystals that often form in the cask, in the sediment and on the corks of naturally made wines. These deposits come from the tartaric acids present in wines; though they look like cut glass, they are totally safe. In fact, they are a positive indication to experienced tasters that a wine has not been overly processed.
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