Rock hunting in Yellowstone - David Kim

Our trip to Yellowstone was exciting though it was completely crowded with people.  As we were walking along, we noticed several black shiny rocks which some in our group said looked like glass.  Due to how hard and brittle it appeared, and the fact that one in our group cut his finger on it, I assumed it would be obsidian.  The fact that it was found in Yellowstone which is a volcanic site helped confirm my suspicions. 

I explained to the group that obsidian formed when lava that has a high content of feldspar and quartz cools rapidly.  Another friend commented that a lot of the rocks we had seen were rhyolite, which was the volcanic form of granite.  I confirmed this by saying that the grains no the rock were so small that they were almost imperceivable which lends itself to the belief that it was in fact rhyolite.  In addition, though it was difficult to see, it looked as if there were tinges of pink which would lead me to further believe that it was rhyolite.