Calcium deposits.

The white dots that people sometimes get in their fingernails have a medical name:"leuconychia" (pronounced loo-ko-nee-kee-a, also spelled leukonychia). The white dots come about as a result of damage to the fingernail as it grows. Basically, if you hit your finger on something it can cause a deformity (often called an "air bubble") in the fingernail. What it really represents is an area where the material that makes up the fingernail (called keratin) is not put together quite right (we say it is incompletely keratinized). This deformity stays in the nail until it grows out and gets cut off. To really understand this, you need to understand how a fingernail grows. Most of your fingernail is "dead", like your hair. But there is a part toward the back, closer to your knuckle, and mostly under the skin, that is alive and growing (push on this part of your fingernail to see how much it hurts). This part of your fingernail is always growing and "pushing" on the rest of your fingernail, causing it to "slide" over your finger. In other words, the end of your fingernail was, at one time, way at the back end under your skin. When you hit your finger on something, you can damage the part of your fingernail that is alive and growing. This doesn’t cause any permanent problems, but causes this white spot in your nail. The more you bump or hit your fingernail, the more white spots you will get. But whether you have one or many, they stay in your nail until it grows out.

I was always told by my mother and grandmother that it meant you were lacking in iron.