The latest rock I collected is this piece of Bishop Tuff, which I collected from near the mouth of the Owens River Gorge in the volcanic tablelands north of Bishop, California. The Bishop Tuff is the hardened ashfall of a stupendous volcanic eruption that took place about 600,000 years ago where the Long Valley Caldera sits today. The tuff is found all over the west as a marker bed, but it's hundreds of meters thick here. Every geologist should have a piece of it.
Bishop tuff Geology Guide photo