How to decide if piano keys are ivory: Hot Needle Test
Use a needle that has a very fine, sharp point. Hold the tip of the needle over a lighter flame until it glows red hot.
Press the glowing hot needle against a part of the piano key that is least visible in case of damage.
If the needle melts a small hole in the piano key, it is made of plastic or some other type of synthetic substance. Unlike plastic, an ivory button cannot be penetrated.
Ultraviolet light and magnifying glass sample
Hold an ultraviolet light over the piano keys. If they are made of ivory, the buttons will glow either white or with a violet blue color (see Ref. 1). If you do not have access to an ultraviolet light, go to step 2.
The piano keys were always made of three separate pieces of ivory; two on the upper surface, where the fingers are placed and one covering the small front section. Use a magnifying glass to look for a very fine line where the two sections on the upper surface join (See ref. 3). If there is no line, the key is probably made of a piece of plastic or other material.
Use the magnifying glass to look even closer. Ivory contains Schreger Lines (See Ref. 1), which will appear as a small pattern within ivory, not unlike a fingerprint.
Tips and warnings
Piano keys were originally made of ivory, not only because of its value and because of aesthetic quality, but also because its properties make it possible to absorb sweat. Thus, ivory keys, unlike plastic materials, can reduce the risk of the pianist’s fingers slipping. However, due to animal welfare laws, ivory has not been widely used in the production of piano keys since about 1950. If you own a piano that was manufactured in the last 50s, it is highly unlikely that it has ivory keys.