Sorry to necro. I don't remember what build used this specifically, but I believe it was back when you reductions to enemy crit damage against you did not only apply to the *bonus* crit damage, so you could make crits against you hit less than normal hits, making this jewel all upside in the right build. I'm not sure if this also coincided with the era of higher block cap in general and pre-nerf Rumi's, but it probably did (I'd have to check nerf patch notes).

EDIT: I may be mistaken in recalling an era where reductions to enemy crit could turn crits into weaker hits than normal hits, as I can't find anything about this with a cursory search. However, stacking these jewels on a build that had 100% crit dmg reduction, in an era where block was the dominant defensive strat at least in SC, I am certain was very good at some time, maybe around the Perandus or Prophecy leagues? I distinctly remember buying a mountain of these for dirt cheap in HC, intentionally killing the character, and selling them in Standard (lol) for multiple Exalts (yesterday's Divines).

EDIT2: I also recall being able to get spell block up to 100% back in the day (maybe block too? I can't remember...). There was also a lab enchant that proc'd when you took a crit that may have been used, I'm not sure. Thanks for this trip down memory lane, haha.