There's also the point that Christ carried the 'cross' to the execution place, and if it were an actual cross, it would likely be too heavy and unweildy to be carried by just one man (Simon takes it to carry on his own during the journey as well). Whereas a stake, although heavy, could more easily be carried.
I think Jesus carried the cross-bar bit of the cross, then it was hooked onto the other half when he got there. That's what I thought, does that make sense?
Anyway, I like my necklace!! lol, but I also do think it has pagan origins