Sorry for the confusion, Stoo. To clear things up, the seller had a bunch of stuff listed, each with a disclaimer that he was listing them as "Roman," which was his best guess, but he didn't really know. He got all the stuff from one person, and didn't have details about where the items were found, or even what country they came from. But he was selling them all as antiquities. And the other items up for sale were all odd bits and pieces of metal from ages past. So I have no reason to doubt his claim.
I do not think it's a fan-made item. Honestly, I highly doubt it's a prop. But when someone else suggested that to me, the idea was immediately exciting.
I believe it is an antiquity. I believe it is a Christian cross of some sort. That final, belief, however, is the most debatable. But, until I or someone else finds another cross like it with some provenance, then your guess is as good as mine.
It is a pendant, and, you're spot on, Stoo. It looks to be just about the same size as Kazim's tattoo. It's really rather large for a necklace. About 2 1/4" x 1" (57mm x 26mm) Just the right size to see it in a medium shot.
However...
I haven't seen cross pendants like this anywhere in the movie. It's bronze and has a heavy patina. Another indication it's a genuine antiquity and not something made for the movie. You can patina something to make it look old, but more likely, if it was for the movie it would have been left bronze or made out of gold or silver.* But if it was made for the movie, my guess would be it was to be used in place of Kazim's tattoo. Join the Brotherhood get your grail cross.
That much speculation makes the 'mislaid, unused prop' theory rather sketchy.
Still, the fact that no one saw crosses like this before Crusade.....
* I wear a Byzantine bronze cross necklace on a daily basis. I never take it off. When my wife gave it to me, it was the same patina as my "grail cross." But after wearing it for a few months, it got back to its original bronze color.