Alkahest is a hypothetical universal solvent, having the power to dissolve every other substance, including gold.

That He might manifest A crimson glory subtly caught and come, An opal crucible of Alkahest!

However, the alchemist Philalethes specifies that Alkahest dissolves only composed material into their constituent, elemental, parts.

P.A. Porto: "Summus atque felicissimus salium": The Medical Relevance of the liquor Alkahest.

Dan then tells Amy that he tricked the Kabras into thinking that Lake Tash was the next location, and the two figure out that it's actually an anagram for the words, Al Sakhet and Alkahest, the alchemical word for the philosopher's stone.