Dave and I had several brief conversations about so-called "wood grain" tarnish, but could never come to agreement on the cause. Attributing it to "imperfect alloying" could not be demonstrated without precise testing, and Dave did not have data. This left toning attributable to some near-final step in planchet manufacture possibly copper rolling prior to blank cutting by Scoville Mfg Co., the supplier of cent blanks.
That this "wood grain" is found among Indian and Lincoln cents from all mints for nearly twenty years, points to a single source. Further there is no mention of any visual defect in new cent color by Mint sources -- it seems noticeable only after passage of time. It is always if the form of linear streaks, indicating it arises from an analogous linear manufacturing process. Last, not all coins are affected implying that there could have been several sets of production equipment, but only one that created the unusual toning conditions.