What is the "Emerald Tablet?"

The "Emerald Tablet" is an emerald inscribed with "the most important text in the literature of alchemy".

In one of the books he learned that the most important text in the literature of alchemy contained only a few lines, and had been inscribed on the surface of an emerald.
"It's the Emerald Tablet," said the Englishman, proud that he might teach something to the boy.