This is such a neat rock. Ever seen obsidian? It’s formed when rock is molten and cools fast enough that it can’t form crystals. The dark material in this rock is very similar – a rock type called a pseudotachylite.
A tachylyte is the term for basaltic glass, typically formed when basaltic lava enters the ocean and rapidly cools. The dark portion of this rock, a pseudotachylite, forms when the rock is rapidly heated and cooled, leaving behind a dark, glassy rock with either very fine crystals or no crystals. This rock started as a part of the Precambrian crust in Colorado. In this case, the rocks are found in a 1.4 billion year old fault zone. The rocks were fractured and heated by friction caused by the faulting.
Image credit: James St. John