Charred blue coals??

[Heston]

I was burning some piles of brush and limbs. Some seasoned some green and some rotted. I was raking the some of the smaller stuff from around the edge of my fire pulling back into fire. When this log flipped over I saw all I can describe it as blue coals or blued charring. Can anyone tell me what this is, how and why this accures. I'm in my 40's and never seen this ever!!

 

[svk]

Elements/minerals that were present in the ground water that the tree consumed. Often yellow, pink, orange, green, or blue. Usually most common in the stump or the first section or two of wood in the trunk above the stump.