Tuffisite (pink) is formed in obsidian lava (black) when the pressure of hot gas in the lava flow breaks the obsidian briefly and the gas and some ash is vented-out. The cracks rapidly anneal after the gas pressure is released and trap some of the ash that then welds to form a thin pink veneer on the obsidian. The thin tuffisite layers are then pulled-apart and disturbed as the obsidian lava continues to flow. See: https://www.frontiersin.org/articles/10.3389/feart.2016.00041/full