Finding what appears to be small glass shavings in your wine bottle may not only be confusing but also scary. The good news is that there is no reason for panic because these small crystals are not dangerous and more of a distraction than anything.
The crystals are caused by tartaric acid and potassium binding together when a wine is chilled. These crystals are clear and look like rock candy or snowflakes on the bottom of the cork. If they dislodge from the cork, they can look like shards of glass in your wine. They are entirely natural and completely harmless, nothing more than a type of sediment.
Most winemakers use a process called cold stabilization to prevent the crystals from forming at your dinner table. Before bottling, the winemaker will chill the wine to near freezing, which causes the crystals to form so they can be removed. Cold stabilization is generally used only on white and blush wines — not because red wines do not form the crystals but because they are rarely chilled low enough to allow the crystals to form.
The crystals, like any other wine sediment, can be removed by decantation. Stand the bottle upright for a few hours to let the crystals settle in the bottom, then slowly pour the wine into a decanter.