The Wiki entry on X describes it thus:

X is the name of a class of compounds created from nitrocellulose and camphor, plus dyes and other agents. Generally regarded to be the first thermoplastic, it was first created as Parkesine in 1856 and as Xylonite in 1869 before being registered as X in 1870. X is easily molded and shaped, and it was first widely used as an ivory replacement. X is highly flammable and also easily decomposes, and is no longer widely used. Its most common uses today are the table tennis ball and guitar picks.

What's X?

Ans: Celluloid