Matte Proof Lincoln Cent "wood grain" toning pattern

What gives?

Over the years I've seen a number of Lincoln Cent pics with this odd "wood grain" toning pattern. I'm curious if there is any specific environmental, storage, minting, handling, metallurgical or other factors that results in this? I really don't find this attractive in fact it throws my OCD into a tizzy (LOL) it's very distracting, especially among a set of relatively even toned Lincolns. BUT...for a key or rarity it would serve as a nice hole-filler...for a while

 

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It's the result of the alloy not being mixed thoroughly.

 

Oddly, some folks find coins of this nature rather pleasing. Just as nuts like me enjoy laser etched 'moderns'.