This mixed formation of popcorn and helictites looks green when a light is shown through it. The green coloration comes from the presence of nickel in aragonite crystals. When nickel is present in calcite the result is a lemon yellow color. (You may have noticed that some of the formations in the Cavern of Sleep were yellow, this is due to the presence of Nickel in Calcite.) Other minerals can color formations as well, iron causes them to appear brown, reddish-orange or even a dirty yellow, while manganese can cause formations to appear black or bluish gray in color.