It is often told that the only material that can be used to taste caviar is mother of pearl, and that metal cannot be used because it oxidises the caviar, changing the taste of the product.

This is not quite right! If this were true, then why are metal tins used to make caviar? The moment we put a metal spoon in our mouth to taste caviar, it leaves a metallic taste on contact with our tongue, changing our sensory perception.

This phenomenon can be identified as parageusia i.e. when an external factor results in a change in taste perception. To prevent this alteration of the organoleptic sensation, therefore, the spoon traditionally used was mother of pearl, even bone, ivory or gold, or it was tasted directly on the hand (when traders valued caviar, there was often only one spoon present).
Alternatively, a plastic spoon can also be used, although mother of pearl remains the best material as it can be washed and reused many times: a personal caviar spoon!

A tasting method that has become a classic is the caviar bump, a technique used historically to assess the quality of caviar without using a spoon.