Why are some cave formations white while others are tan, orange, or brown?

Cave formations, known as speleothems, are primarily composed of calcite, a form of calcium carbonate. In their purest state, these deposits are white, though they often appear milky due to tiny inclusions of water trapped within their crystalline structure.12

The variation in color—ranging from tan and orange to brown—is caused by impurities introduced by seepage water as it travels through the environment. The most common coloring agents are:1

• Humic substances: These organic products derived from plant decay in overlying soils are the primary source of brown and tan hues.1
• Iron oxides: Minerals such as goethite or other iron-bearing compounds carried into the cave by water can impart shades of orange, red, or yellow to the formations.123

These pigments are incorporated into the flowstone and other deposits as the water evaporates or loses carbon dioxide, causing the minerals to precipitate out of the solution and build up over time.12