When I finished a glass of wine recently, I noticed little grains of some sort in my glass.  When I looked at the bottle, there were grains of sand or something inside the bottle too.  Where did these come from and are they safe?

--Pat

What you saw are likely tartaric acid crystals that precipitated out of the wine.  Tartaric acid is naturally found in grapes and other fruits.  The chemical name is potassium bitartrate with a formula of KC₄H₅O₆.  You know it better as Cream of Tartar, a common culinary ingredient in baking, used for stabilizing beaten egg whites, or for leavening when combined with baking soda.

Like the formation of any crystal that precipitates out of a solution, tartaric acid crystals need some sort of nucleation point to begin forming.  That is why you may find them attached to wine corks or the inside of wine casks.  Both of these have rough surfaces that encourage the crystals to form.  The crystals may also form on any microscopic impurity in the wine, such as a bit of yeast that wasn't filtered out, or other organic matter.  The crystals form more readily in wines stored below 50ºF (10ºC), but will not redissolve readily, at least at the temperatures that are good for the wine.

Tartaric acid crystals are sometimes mistaken for broken glass or sand that has somehow gotten into the wine.

In the quantities you would have seen in your wine glass, it is safe to ingest.  It is also sometimes called wine diamonds.