Beat the main quest and shop quests to become master, just wondering what the head mantle thing is behind theguild masters table by the other treasures? Is this possibly a spot for gray foxs mask??? (wishful thinking)

It's for the crown of Barenziah.
http://www.uesp.net/wiki/Skyrim:No_Stone_Unturned

You need to collect all 24 of the "unusual gems."

It has to do with a certain quest.

Edit: Ninja'd

EDIT: Damn, double ninja'd.

Baren....awww crap

Don't do it, man... you'll be up to your elbows in gems.
I eventually had to remove 50 weight of them (using the console!) which is like 500 gems.
It gets so ridiculous, it's farcical...

Makes a mockery of the game's economics engine.
Note: you don't experience all the wealth early in the game (when you really need it).
No. By the time you collect all the stones (and there's the guy's popular mod, which puts a map marker on them all, which helps) but by the time you collect them you're going to be a very senior player. It's a late-game thing.

Anyone who's going to do it should get the guy's mod, and do it early. Great time for fabulous wealth.
But my guy now won't touch those Unknown Gems, heheh... let 'em sit.
Crown does look sweet when you put it in the TG though.