You tilt a glass to get that last sip, and as you watch the elixir disappear, you notice a rivulet of tiny, sparkling, diamond-like crystals tumbling towards your mouth. You recoil from the glass, spitting and sputtering.
Then, you open your laptop and email your closest wine blogger/writer/aficionado asking what this stuff is.
Of all the things in wine, these tartrate crystals are among the least harmful. Tartaric acid occurs naturally in grapes and is the form of acid added during wine making, if the grapes are deficient in acids.
With time, these acids precipitate out of wine – particularly as the temperature of the wine drops. As a matter of fact, chilling wines for several weeks is one of the ways of dropping the acid levels in wines that are too sour.
Wines that have been stored for several years, especially in cooler temperatures will have a sediment, part of which is composed of tartrates, which, in turn can, form crystals.
I am not sure that it can reliably be said that the presence of these crystals means anything more about the wine other than that there was enough tartaric acid in the wine, and/or the wine was exposed to sufficiently low temperatures for long enough for crystals to form.
These crystals are neither unpleasant or interesting to taste. They just look weird and can be gritty in the mouth – perhaps the main reason for learning to properly decant aged wines.