A crucible is a melting pot used for extremely hot chemical reactions — the crucible needs to be melt-proof.
Literally, a crucible is a vessel used for very hot processes, like fusing metals. Another meaning of the word is a very significant and difficult trial or test. Scaling Mt. Everest with your legs tied together would be a crucible, as would swimming the English Channel blindfolded. Whether or not to have children is a crucible for many people. The title of Arthur Miller’s play The Crucible is an example of this meaning.