Crystals are having a moment right now. Over the anxiety-inducing course of the last two years, the concept of personal wellness has entered the forefront of many people’s minds and has since manifested in many ways. One such way is the resurgence of crystals, which have long been said to have healing properties from natural earth elements. The popularity of healing crystals has exploded in recent years with the dominant markets, millennials and Gen Z, using the stones in hopes of improving their quality of life and mental state.

Those who seek self-care in other ways, say with a glass of wine, may be shocked to find crystals, or “wine diamonds,” under the cork of their bottle once the corkscrew makes that satisfying pop. We asked VinePair’s own wine pro, tastings director Keith Beavers, why these harmless crystals often appear under our wine corks. The answer boils down to one essential component of wine: tartaric acid.

“There are two very important acids in wine — malic acid and tartaric acid,” says Beavers. The latter is responsible for the appearance of wine crystals. Though tartaric acid is used to maintain a wine’s body, character, and color, many wines contain a surplus of the acid, which accounts for the aforementioned “diamonds.” “Potassium, which is another big element in wine, attracts the acid and binds to create a salt called tartaric bisulfate,” Beavers explains. “This is soluble, so it separates itself from the wine and creates this clear salt, or crystal.”

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The color of these crystals can be directly attributed to the pigment of a specific wine. “In red wine, they can often look brown or dark red. In white wines, it can look brownish, or sometimes, it looks like glass,” Beavers says. The reason they can often be found on the cork of the wine is simple: Some wines are laid on their sides during the storage process to keep the cork intact and from drying out. During the aging process in these bottles, the cork is therefore the easiest place for crystals to attach. Sometimes, these crystals can fall to the bottom as well when the wine is reoriented onto either its side or right-side-up, creating something known as sediment.

Due to the unpredictable nature of tartaric acid, it is impossible to predict which types of wines are most likely to form crystals or sediment — it can happen in both reds and whites — but Beavers stresses that these crystals are harmless. He explains that it is not possible to completely eliminate the possibility of crystal formation because “science happens when science happens.”

The concentration of tartaric acid depends on a multitude of factors going beyond the types of wine it is found in. For example, a Merlot produced in Bordeaux can have wildly different levels of tartaric acid from a Merlot produced in Napa, despite the fact that they are made with the same grape variety. Weather, region, nutrients in the soil, and grape varieties all play a role in determining the amounts of acid present in a particular grape. For example, there is more tartaric acid in cool-region wines than in warm-region varieties.

Older wines are especially likely to form crystals or sediment once the cork pops off. As wines are constantly evolving during the aging process, elements within the wine are constantly interacting with each other. As such, some chemical bonds that are formed in the bottle are denser than the wine itself, causing them to sink to the bottom of the bottle. All of this, Beavers says, is a natural part of the aging process. “People should understand that if you’re buying an old bottle, there is going to be some sediment.”

Beavers also says that, while uncommon, crystals and sediment can also appear in young wines — but if they do, it’s typically indicative of “messy” winemaking rather than a chemical reaction occurring in the aging process. “If it’s for everyday wine and you see crystals, it’s not terrible, but it might not be a great quality of wine,” he says.

While wine crystals and sediment still appear in our bottles today, they were once far more common. Today, there are practices winemakers can employ, like cold soaks, to rid the wines of as much tartaric acid as possible during the production process and therefore limit the possibility of sediment formations. Once the wine is fermented but has not yet been bottled, winemakers will bring it down to a low temperature that forces out tartrates. The wine is then filtered and bottled for consumption.

While wine crystals are harmless, many drinkers may not want to see them in their glass. So, if you prefer getting your crystals from your local wine shop rather than the earth shop, try straining and decanting your wine to remove any unwanted “diamonds” or sediment.