Turgite is a name that is used for two minerals that grew together underneath certain environmental conditions. The "rainbow" is a naturally iridescent iron oxide coating upon the "mineral" of goethite and hematite and both are in the iron family.
Turgite can come in a couple of colors which are black or red.
Goethite: Oxide minerals of the sydroxide subgroup
Hematite: Oxide minerals. Iron oxide
Origin: Graves Mountain, Georgia (known for its attractive turgites)
Weight: 23 grams