http://www.etymonline.com/index.php?term=crucible says:
early 15c., from M.L. crucibulum "melting pot for metals," originally "night lamp." First element might be M.H.G. kruse "earthen pot." Or perhaps from L. crux on some fancied resemblance to a cross. Used of any severe test or trial since 1640s.

But maybe it's not a physical "fancied resemblance" so much as a vague metaphorical resemblance.