You may be heard of the phrase “tickling the ivories,” which refers to playing the piano – with the assumption that the top of the keys were made of ivory (the rest is wood). Ivory, which comes from animal tusks such as elephants, was chosen because of its esthetics, its hardness, and its ability to absorb the perspiration from a performer's fingers. However, ivory keys are more likely to chip or crack, and are unlikely to be replaced by ivory as its use has been outlawed.
One of the ways that you can identify an old piano is by examining its keys. Over time, ivory keys turn yellow; the darker the shade, the older the ivory keys. Moreover, the keys' colors may vary from one to another on a piano as each key comes from natural tusks with their own variations.
Even though keys can be bleached, the coloring is likely to be unique. Ivory keys also have natural patterns like finger points, which a magnifying glass might reveal as cross-hatches, curved lines, or diamond patterns.
Speaking of color, an ultraviolet light will reflect a bright white or blue-violet color, whereas a plastic key will appear dull because it absorbs the light.
Another way to identify real ivory is by seeing if there is one than one piece of key per key. Two pieces of ivory made up the top layer, and another piece made the front edge. So you can use a magnifying glass to see where the pieces join.
Although not encouraged, you can use a noodle to test ivory. If you heat up one end over an open flame until it glows red hot, and then press that end on the key, if it melts or dents the key, that key is NOT ivory.
Ivory is not permitted to be made into piano keys now; instead, keys are generally made of plastic, which is cheaper, more consistent looking, easier to work with, and less prone to damage (especially to the animal). High-quality plastic eys can be hardly distinguisable from ivoty.
You can still BUY a used piano with ivory keys, but not a new one. Furthermore, an antique piano with ivory keys is supposed to have along with it the appropriate paperwork and permits for transfer of ownership and shipping. Buyer, beware.
