sedimentary

Pyrite Concretion (Grapestone)

Iron Sulfide (FeS2) Concretion

Hardness: 6-6.5 on Mohs scale; Color: Brass-yellow to grayish-brown (when oxidized); Luster: Metallic to dull; Crystal structure: Isometric (often cubic or botryoidal clusters); Cleavage: Indistinct; Specific gravity: 4.9-5.2.

Hardness: 6-6
Color: Brass-yellow to grayish-brown (when oxidized)
Luster: Metallic to dull

Identified May 23, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of mineral cement within the spaces between sediment particles. These often grow around a nucleus such as a fossil or pebble in anaerobic sedimentary environments, typically during the Cretaceous or Jurassic periods.

Uses & applications

Primarily of value to mineral collectors and as educational specimens. Historically used as a source of sulfur or 'firestone' to create sparks.

Geological facts

This specific 'grapestone' or 'pineapple' shape is a common growth habit for pyrite and marcasite concretions found in clay and chalk beds. While they look like man-made artifacts or fossils, they are entirely inorganic geological formations.

Field identification & locations

Look for heavy, metallic nodules in sedimentary layers like shale or chalk. They are often found on beaches where cliffs are eroding. Identification in the field is based on their high density and distinctive 'grenade' or 'grape' like segmented surface.

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