So, you've now arranged things such that everyone has the opportunity, playing leisurely, over time, to claim all the special prizes from a Feat, in the form of small petrified rocks. That's really cool. Here's the problem: medium and large petrified rocks are themselves special prizes from a Feat.
I have two large rocks which were fairly difficult to get, compared to most everything else I've ever gotten in Glitch. It's quite possible that I'll never earn another one. Now I can crack them open... or I can keep them. Or sell them. I've seen small rocks -- small rocks! -- traded for hundreds of thousands of currants. Imagine how much a medium rock would sell for. Imagine there are (at least for the immediate future) fewer large rocks in the game than there are Highest Quality #1 "Imported" Glitchmas Yetis, and what exactly is my motivation to exchange mine for a few pieces of an ancient artifact that anyone and everyone will get in the long run?
You've successfully eliminated the super-special-hardcore-rewarditude from the artifact pieces, but now you need to remove it from the medium and large rocks themselves.
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Proposed solution #1: Medium and large petrified rocks could automatically grind after a certain amount of time.
This would eliminate any advantage that could only be gained by outcompeting everyone else. The only exclusive benefit is a one-time iMG boost for "discovering a new useful item", and this isn't visible to other players at the moment. One issue would be dealing with large rocks that have been stored away in SDBs. Possibly remove those from the SDBs and give the contents to the magic rock for safekeeping.
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Alternate solution #2: Medium and large petrified rocks could be soulbound.
This keeps these rocks off the market, but still allows the winners to grind them at their choosing if they wanted to sell their prizes. This doesn't prevent players from showing off their rewards in towers, which could be thought of as an advantage (as that's about all the artifacts are good for as well) so this solution isn't perfect by any means.
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Problematic solution #3: Instead of medium and large petrified rocks, award a random assortment of artifact pieces (as was done for "Mihceal, Blow that Conch") along with a couple small rocks.
This would seem like the best option to me, except that by discontinuing medium and large rocks the situation would be even worse than it was before the start of the Epic. So this suggestion could only be implemented in combination with solution #1 or #2.