Chert with Asphaltum (or Tar)

Physical Properties

Hardness: 7 (for the chert base), 1-2 (for the bitumen); Color: Tan/yellowish base with black coating; Luster: Waxy to vitreous base, dull to resinous coating; Structure: Microcrystalline; Cleavage: None (conchoidal fracture on base).

Formation & Geological History

Formed as a chemical sedimentary rock through the precipitation of silica. The black coating is natural bitumen (asphaltum), which is a semi-solid form of petroleum that seeped through geological fissures and adhered to the stone.

Uses & Applications

Used primarily for geological study or as a curiosity specimen. Historically, bitumen was used as an adhesive, sealant, and for waterproofing.

Geological Facts

This specimen represents a natural interaction between common silicate rocks and liquid hydrocarbons. Natural tar pits, like the La Brea Tar Pits, produce many such 'oiled' geological specimens.

Field Identification & Locations

Identify by the contrast between the hard, waxy silicate rock and the soft, sticky or brittle black carbonaceous coating. Often found in proximity to oil seeps or sedimentary basins. It will often have a faint kerosene smell if rubbed.

Identified on: 4/23/2026

Mode: Standard