This is a very common question and it’s not as easy as it might seem. There are actually quite a few ways to tell if your piano keys are made of ivory.
Generally speaking, ivory keys are not one piece on the top; they are most often made from three separate pieces. There are two pieces on the top of the key and one on the front side. With most ivory keys you should be able to see a very fine line that separates the front and back piece on the top of the key.
Another telltale sign of ivory keys is that eventually they will begin to yellow. Keeping your fallboard open will help to alleviate this some because light helps to bleach the ivory. However, plastic keys can also yellow and in fact some are produced with a yellow tint to appear more like ivory key tops.
If you have a key that has one piece along the top; it might still be ivory. If you look very closely at an ivory key top you will notice a pattern that looks a lot like a fingerprint. This is because they are made from elephant tusks. You will need to look very closely – you might need a magnifying glass and a really good light source. Look for the fine pattern to determine if they are real ivories or not.
Ivory keys are porous, so they will have a texture to them as well. When you run your finger across them, you can tell a difference between the feel of plastic and ivory keys. However, some ivories are polished and will feel completely smooth. Also, some plastic key tops have faux patterns to look like ivory.
One indicator of whether or not your piano keys are possibly ivory is when the piano was produced. If a piano was made after 1972, chances are you don’t have ivory keys because the ivory trade was made illegal around that time. However, pianos were produced with ivory or plastic for many years prior to 1972. There are still sources of ivory keys available rarely today, but they are very expensive, and possibly illegal.