“Gravelle” is the name given to a deposit (often found on the bottom of some wine bottles) that resembles very fine sand. Although it may seem unpleasant to the inexperienced people, this deposit is particularly noticeable in white wines and shouldn’t be viewed as a flaw. They pose no danger to the consumer. They are actually just crystals of potassium bitartrate, a byproduct of tartaric acid, an organic acid found in grapes that precipitates when exposed to cold. Normally, this deposit forms in the barrels throughout the winter, but occasionally, some of the potassium bitartrate is not completely insolubilized.
Because of this, the wine will continue to be saturated with the salt, which will eventually settle in the bottle. Since tartaric acid causes a portion of the wine’s acidity to be lost, the precipitation of potassium bitartrate (also known as “crème de tartre” in French) enhances the wine.
It should be noted that this deposit is often removed by means of a cold stabilization before bottling. This will accelerate the precipitation of the crystals and to separate them from the final wine.