I believe unique gambling was removed entirely in 1.07, and added again in 1.09, with very low chances. Apparently, it's only about a .009% chance of getting one. That's about 1 in 10,000. So the chances are extremely low now. We always just gamble amulets (and circlets) since these can be useful if rare. If we had gotten a lot of unique amulets that way, we might consider switching to rings, for a chance at a SoJ. But that just isn't happening. We still haven't found or gambled a SoJ.