When opening and pouring some of your aged wines, you may see small crystals on the bottom of the cork or in the bottom of your glass or bottle. These completely harmless particles are known as wine diamonds or tartrates.
When the wine kit juices are shipped to the kit facility, they are pumped into nitrogen-purged tanks, tested for quality and stability and held at very low temperature. This both speeds up the formation of wine diamonds (crystals of potassium bitartrate from the tartaric acid naturally occurring in the wine) and preserves the liquids.
However, sometimes not all these crystals are removed and can still appear at a later date. If you have ever used cream of tartar for baking, you have actually used the very same tartrates.
The best serving technique for wines with wine diamonds is to decant the final part of the bottle or use a filter. If wines are aged for sufficient periods of time or stored in too cool an area (remember ideal storing condition is 15C) these crystals can still precipitate out.
Keep in mine, wine crystals are not a bad thing, and they can actually have a mellowing effect on the wine.