Post from the forums about a guy who ran cosmic Kurse 400 times and still no medaliion, Doomsaw's response:
"To provide some insight - looking at the loot tables, Gem of the Kursed is the rarest of all the specific boss drops (even rarer than Hand of Doom), so it's possible you could get it on your first kill or it's possible it could take several hundred kills.
I think it's still a little too rare, frankly, but it's much better than it was 24 hours ago.
Depends on your rarity boost, but I believe it's 1 in 200 and Hand of Doom is 1 in 100.
With numbers like that, there will be people who get 10 drops in 100 runes and people who get 1 drop in 1000 runs.
I'm going to time the difference in the two fights this weekend and see if we can up the Kursed one based on length of fight compared to Doom. Volunteers to do some tests would be welcome as well."