Billiard balls have been made from many different materials since the start of the game. For example, they have been made from clay, wood, ivory, plastics (including celluloid, Bakelite, crystalate, and phenolic resin) and even steel. The most common substance from 1627 until the early- to mid-twentieth century was ivory. The search for a substitute for ivory use was not for environmental reasons but based on how expensive they were and fear of danger for elephant hunters. The search was made more enticing when a New York billiard table maker offered a $10,000 prize for a substitute material. The first usable substitute was made from a material called "celluloid". Celluloid, which is an early form of plastic, was invented by a man named John Wesley Hyatt in 1868. There was a problem with the material though. Celluloid was unstable and highly flammable, sometimes exploding when people were making it.