What are piano keys made of? Are they made of plastic (whites) and ebony (blacks)?
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This answers the question for most acoustic pianos:
White piano keys are mostly wood with plastic tops and fronts to make them look and feel similar to ivory, which used to be used for the tops and fronts of the white keys.
Black piano keys are usually made from a dark hard wood that has been stained to make it black so it looks and feels like ebony, which keys used to be made from. Many black keys today are made from actual ebony, or at least wood from the ebony family, but since a lot of actual ebony is not evenly and fully black, it is often stained black in any case.
Digital piano keys are made from entirely plastic at the low end of the price scale, with more expensive pianos having partially wooden keys, and the most high-end of digital pianos will have keys of the same construction and size as acoustic keys, and use reduced or faux actions and sensors to detect key/action motion to trigger the sounds.
If you're playing an old enough acoustic piano, the white key tops may be worn down showing layers, and they may have a partly yellow or brown tinge. This is a sign that they were topped with actual ivory. Another way to detect actual ivory is by feel, but many modern plastic formulations for key tops are meant to feel like ivory. I have not personally had a chance to compare actual ivory to imitation ivory plastics - it's been many years since I played on a ivory piano.
Compulsory Paul McCartney & Stevie Wonder link coming up...
Ebony & Ivory
These days, of course, mainly plastic - as both elephants & ebony are protected species.
Wikipedia - Piano [construction] Ok, it was funny. Ignore this: I can't remove the DV without editing.