Is there a reward for completing challenges?

I binged the Sharpstone Keep 2x coins challenge on Monday and Tuesday and maxed out the coin total. Then the next day I got a reward chest in my messages, which I wasn’t sure what it was for. Do you get a reward for maxing out a challenge?

If so, how long before it drops? I finished the 2x gear reward yesterday too.

I’ve never once got a reward for finishing the challenges, though I only use my free attempts, if it costs gold I may do it again, gems not so much

I did the Sharpstone coin challenge because the rewards were terrific and it kept me under 250 gems so Mirt wouldn’t “help” me revive. It felt like over half the rewards I got were coins and the 2x bonus made that challenge very very lucrative. I can also clear Sharpstone for 3-dice easily, which is why the new challenge in the mines isn’t quite as much fun (I get 2 dice, then get whomped almost immediately after).

However, it doesn’t look like completing a challenge gets you any kind of bonus so I’ll save the gems.