Metaphorically, an environment where someone is transformed by pressure is a crucible, defined by OALD as
- (formal or literary) a place or situation in which people or ideas are tested severely, often creating something new or exciting in the process
In the literal sense, a crucible is a vessel in which a substance is subjected to intense heat, as for liquefying metal to extract it from ore, or for combining them to produce alloys, from which the sense of a severe test, especially a transformative one, naturally follows. Early OED examples include
1647 · J. Howell New Vol. of Lett. 106 In the limbeck and crusible of affliction.
1796 · H. Hunter tr. J. H. B. de Saint-Pierre Stud. Nature (1799) III. 332 A ship is the crucible in which morals are put to the test.
Arthur Miller's 1953 play The Crucible refers to the figurative crucible of Puritan Salem, Massachusetts, not to any physical object.