Back in that time, I presume that a night lamp would either have been a candle or an oil lamp, perhaps something like these http://en.wikipedia.org/wiki/Oil_lamp
or these
http://www.romulus2.com/lamps/lampcat/lampcat1.shtml

Here's a nice blow up of how this works:
http://en.wikipedia.org/wiki/File:Diya.jpg

There's also a kind of fungus called crucibulum that has a pretty clear resemblance to these lamps.
http://en.wikipedia.org/wiki/Crucibulum

There's a kind of gastropod called crucibulum:
http://www.gastropods.com/0/Shell_430.shtml
See the resemblance there.

The resemblance of the modern crucible, of the fungal crucibulum, and of the animal crucibulum is obvious - it's just that leap back from lamp to cross seems flimsy to me - unless we think of the metaphorical connection of "great trial." In this sense the crucible was the part where the metal collected, after it was extracted from (melted out of) the ore. This is a kind of purification. Maybe there's some connection with Jesus on the cross, leaving the scoria of his human form to become fully a god. The problem with this hypothetical connection is that use of crucible as a melting pot for metals came after and was derived from its use as a lamp - and it's the lamp that bears less resemblance to a cross. OTOH, there's a different potential metaphor there in John 8:12 "I am the Light of the world; he who follows Me will not walk in the darkness, but will have the Light of life."

Maybe makes more sense, but it's still flimsy. Can't help thinking that maybe there's some lost Latin word that has coincidentally similar spelling.