The following is taken from http://www.yellowstonenationalpark.com/blacksand.htm:
Black Sand Basin, an isolated group of the Upper Geyser Basin, was originally named the Emerald Group by A.C. Peale in 1878. But turn of the century tourists began calling it Black Sand Basin because of the small fragments of black obsidian sand which cover portions of the basin.
Black Sand Basin contains a small collection of jewel-like geysers, and colorful hot springs. Emerald Pool is the most colorful and famous of these springs. It is a deep emerald green fringed by an outer ring of yellow and orange. Another colorful pool is Opalescent Pool. This recently formed pool inundated a stand of lodgepole pine, creating a stand of white skeletons amidst a rainbow-colored pool. An unusual geyser formed on the bank of Iron Creek. Cliff Geyser formed a rim or wall-like ridge of sinter around its crater from which it erupts 30 to 40 feet high.
The famous Handkerchief Pool was once the drawing attraction to Black Sand Basin. Turn-of-the-century tourists dropped their handkerchiefs into this small spring. Convection currents then whisked their laundry away where it would reappear again at the surface, freshly laundered.
Hydrothermal features are fragile rarities of nature. Yellowstone preserves the largest collection of hydrothermal features on the planet. You have an unparalleled opportunity to view hot springs, geysers, mudpots, and fumaroles in a natural setting.
Change takes place naturally in a hydrothermal area, but people can disrupt these processes and cause irreparable damage. Rocks, sticks, and other objects thrown into a hydrothermal feature may be permanently cemented in place, choking off water circulation and ending all activity.
For the sake of all who follow, never throw objects into any feature. Stay on established walkways for your safety and to protect fragile formations that have formed over thousands of years.
It is illegal to collect any natural or cultural objects or to remove, deface, or destroy any plant, animal, or mineral in Yellowstone's hydrothermal areas. Bring drinking water; take out all trash.
While viewing or photographing the area, protect your camera, glasses, and binocular lenses from hydrothermal heat and stray.
Toxic gases exist in Yellowstone. Dangerous levels of hydrogen sulfide and carbon dioxide have been measured in some hydrothermal areas. If you feel sick, leave the location immediately.
Help preserve Yellowstone for the future.