Concretion
Rock
Also called Nodule, Moqui marbles, and Shaman stones
About it
Hard, rounded lump formed as mineral cement grew outward through soft sediment, often around a fossil or shell. Shapes range from marbles and discs to big boulders like those at Moeraki, New Zealand, and Rock City, Kansas.
- Type
- Rock (sedimentary)
- Group
- Sedimentary (concretion)
- Colours
- grey, brown, rusty red, tan, black
- Transparency
- opaque
Typical values: real specimens vary. For more, see mindat.org.