I last showed these stamps in 2013 at the cusp of a Government shutdown. As I said then:
This is a group of four stamps from a larger set. Most of the photographs were taken by Lewis Hine, a famous photographer. According to Wikipedia, "Lewis Wickes Hine (September 26, 1874 – November 3, 1940) was an Americansociologist and photographer. Hine used his camera as a tool for social reform. His photographs were instrumental in changing the child labor laws in the United States."
From the left: Uranium ore, quartz, aragonite, and malachite, on stamps from Benin. I last showed these stamps in August of 2011. Visit See it on a Postcard this week to see who else is playing! Viridian