I've seen, and owned many of these. The materials I have seen include brass, aluminum and "German silver" which is a nickel alloy, NOT actual silver.
The skull and crossbones sometimes seen below the Corpus' feet has two significances: 1. Jesus' triumph over death and the grave. 2. Supposedly the hill of calvary is directly over the grave of Adam (yes that adam) and thus symbolizes Christ's death in atonement for Original Sin.
Above the Corpus of course is the titular. INRI ("Iesvs Nazarenvs Rex Iudaeorvm" ) or "Jesus, Nazarene, King of the Jews"
The round symbol on the back is of course the crown of thorns. In many of the crucifices of this inlaid type, the 'nails' go all the way through, from front to back, so that they may be individually reverenced when the crucifix is turned over.
I don't know what the inlay is made of, but the ones I have seen, they *all* look as if the inlay had been a liquid, poured into the metal frame, and then embellished while the substance was still semi soft. Like a heavy plasticized clay. This may be an imitation of the silver and ebony originals, which of course are a way to reverence the wooden nature of the crucifix.
Such inlaid crucifices are often called "Nun's Crucifix" or "Priest's Crucifix" althought they may be worn or carried by anyone.
The age is usually Victorian or early twentieth century. I do not think they make them now. They are certainly VERY striking.
"Pie Iesu Domine! ~ WHACK! ~ Dona eis Requiem! ~ WHACK!!"
https://www.youtube.com/watch?v=YgYEuJ5u1K0
SageGrouse