i finished this questline a few days ago just to see how good the reward was. the reward is finding more gems in loot. by more, to me, seems more like a lot. after completing my first dungeon i had gathered the following gems:

Amethyst-12
Garnet-9
Diamond-12
Ruby-9
Sapphire-4
Emerald-9

Flawless Amethyst-12
Flawless Garnet-5
Flawless Diamond-9
Flawless Ruby-3
Flawless Sapphire-5
Flawless Emerald-4

i was so amazed at how many gems i was finding. i had 15k gold worth in the diamonds alone. if i had known before that i would find so many it would have been the first quest done for me. kinda svcks i did it on my 48 thief who has 120k gold so i didn't need them.

Good lucky selling it all, no one has that much $

Unfortunate part is the quest at the embassy kind of goes along with that so its very difficult to make that the first thing you do. I would of loved to be able to get that early, but I guess its best left till later in the game so its not too easy in the beginning levels. Its bad enough I like to run to the Steed Stone early in the game, I couldnt imagine how mundane it would be to be rich that early in the game.
I had no idea how powerful that crown actually was, I thought it was like a tiny boost so you would end up getting like 1 or 2 extra gems if they were in a chest but not like that. Gives me an urge to go and find the rest now though lol

i had the same thought too thats why i never did it before.

edit: i did this quest at a high level too so that may have had an effect on how many gems and what kind i was finding.

I'd think it would tend to require a fairly high level (unless you do a whole lot of wood chopping I suppose) in the process of saving up the money to get the one in Solitude. I thought I would try to get it early-ish: I'm level 27 and still haven't saved up enough money, though I'll be getting close soon.