An aramanth log (purple) makes 6 eucalyptus planks (pink) in the TE sawmill.

Confirmed via a minimal pack assembled via CurseVoice launcher, with NEI, WAILA, Thermal Expansion and dependencies, and Natura and dependencies.

Caused by "amaranth" not being found in Planks.textureNames, so findPlankForName("amaranth") returns -1, which then gets corrected to 0 when set as the plank ItemStack's meta. The texture name for amaranth seems to be "purpleheart".

Changing this line to:

addWood(NContent.rareTree, 2, "purpleheart");

would probably fix it.

Seems logical to me, good find.