Sediment and Tartrate Crystals
Ever pull a cork on a bottle of wine and see what looked like broken glass glued to the bottom of the cork? Some people get freaked out about it but the fact is, it's totally harmless. Others think it's sediment, but they would be incorrect as well.
First, let's deal with tartrate crystals. They are formed when a method called cold stabilization is performed on a wine. Cold stabilization is often done in white wines to remove excess potassium bitartrate, a natural substance found in grapes also known as "cream of tartar." White grapes contain fairly large amounts of potassium bitartrate. If most of it is not removed, the wine will form crystals when placed in the refrigerator. These tartrate crystals, sometimes called wine diamonds, will either cling to the underside of the cork, if the bottle is on its side, or fall to the bottom of the bottle, if it's standing up, where they appear to be ground glass to the uneducated eye.
To remove excess potassium bitartrate before bottling, the wine is placed in a stainless steel tank and its temperature dropped to about 30 degrees F. The wine is held at that temperature for a period of approximately 2-3 weeks. Excess potassium bitartrate will then crystallize and drop to the bottom of the tank where it is removed either by filtration or by pumping the wine out of the tank until the hose is just above the bottom where the crystals have formed. The "cold stabilized" wine is then bottled. Some wineries prefer to skip this step, feeling that the process detracts from the wine's flavor and nuance. Having the tartaric crystals appear in the wine means nothing more than it was not cold treated and, thus, may be of higher quality. The crystals have no taste, just a little crunch if you eat them.
The sediment in wine bottles is completely different. The tumultuous act of fermentation presses the grapes to where parts of the stem, seed and skin are pulverized to the point where they become so small they disappear. Seemingly, that is. The fact is, they are just suspended in the wine. As the wine ages, these tiny particles begin to attract each other and form a small mass that is now visible to the naked eye. The longer the wine ages, the more chance they have to come together and eventually will either form particles which fall to the bottom of the bottle or will actually cling to the side or bottom.
Sediment is usually very bitter which is why the bottles should be decanted or filtered to remove it, though most filters aren't fine enough. Multiple layers of cheese cloth may do the trick if you're filtering wine sediment, but not always.