sedimentary
Limonite Concretion (Goonie or Rattlestone)
Limonite (FeO(OH)·nH2O)
Hardness: 4-5.5; Color: Yellowish-brown to dark brown/black; Luster: Dull, earthy to sub-metallic; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3.
- Hardness
- 4-5
- Color
- Yellowish-brown to dark brown/black
- Luster
- Dull, earthy to sub-metallic
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Physical properties
Hardness: 4-5.5; Color: Yellowish-brown to dark brown/black; Luster: Dull, earthy to sub-metallic; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3.
Formation & geological history
Formed by the precipitation of iron oxide minerals from groundwater within sediment. These minerals act as a cement, binding sand or mud into hard, nodular masses. Often found in sandstone or shale layers.
Uses & applications
Primarily of interest to mineral collectors and hobbyists. Historically used as a low-grade iron ore and as a pigment for ochre paint.
Geological facts
When hollow, these are sometimes called 'rattlestones' because loose material inside can make a sound when shaken. They are frequently mistaken for meteorites or fossils due to their unusual shapes.
Field identification & locations
Identify by its bumpy (botryoidal) texture, heavy weight for its size, and the distinct brownish-yellow streak it leaves on unglazed porcelain. Commonly found in eroded creek beds or sandstone outcrops.
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