If you ever see something that looks like little shards of glass in your wine, rest assured they are in fact a completely harmless substance known as wine diamonds.
It’s admittedly a rather romantic name for what is otherwise a banal by-product of winemaking. Alternatively known as tartrates, argols, tartes (in French) or weinsteins (in German), wine diamonds are naturally-occurring crystalline deposits of potassium acid tartrate (the potassium salt of tartaric acid). These crystals form during the winemaking process, adhering to the sides of fermentation tanks and oak barrels. Cream of tartar, a common baking ingredient, comes from these crystals; they are also used as an acidulant (something that makes things more acidic) in non-alcoholic drinks and foods. They also have scientific applications as part of Rochelle salt, which is used in electroplating solutions. The wine industry is the sole source of commercial tartrate crystals and many wineries capitalize on this by harvesting the crystalline tartrate encrustations that form on their winemaking vessels.
Tartrate crystals are harmless, but their presence in a glass of wine can be alarming to those who aren’t sure what they are looking at: they resemble large sugar granules but can also be mistaken for chunks of glass. This is why they are not nearly as common as they used to be, as the majority of the wine industry has opted to remove tartrates from wine completely instead of hoping their customers have knowledge on the subject.
Cold stabilization is used to remove tartrate crystals from wine. This winemaking process involves chilling wine down to somewhere between -5C and -10C for a couple of weeks, which causes the tartrate crystals to precipitate out of the liquid; the wine is then filtered before bottling. Occasionally charcoal or bentonite clay is mixed into the wine at the beginning of the process, which helps tartrate crystals form much more quickly by providing them something to stick to; this significantly reduces the time that the wines must be chilled before the crystals form.
You are most likely to spot tartrates in pale wines like whites and rosés, simply because they are easier to see—they occur just as often in reds. Wine diamonds are also more common in minimally-processed wines, usually those made on a small scale by winemakers that have embraced a more natural winemaking philosophy. Tartrates are fairly common in white wines from cool climates, especially those in Old World regions; German Riesling and white wines from Alsace are prime candidates for spotting tartrates.
If you do notice some tartrate crystals in the bottom of your wine glass (or sometimes even clinging to the underside of the cork), rest assured that it means your wine was treated gently instead of being manhandled. While I haven’t encountered any definitive proof of this, I do believe that tartrates are a sign of superior-quality wine—or at least wine that tastes a little more natural. You’ll never find them in mass-produced bottles, and that fact alone is quite telling. V