igneous

Weathered granite or granitic pegmatite cobble

Granite/granitic rock, composed mainly of quartz (SiO₂), feldspar, and minor biotite or other dark minerals; the reddish-brown areas may be iron-oxide staining or weathered accessory minerals

AI-generated identification · may be incorrect

Identified August 7, 2026More igneous →

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At a glance

Common name: Weathered granite or granitic pegmatite cobble
Scientific name: Granite/granitic rock, composed mainly of quartz (SiO₂), feldspar, and minor biotite or other dark minerals; the reddish-brown areas may be iron-oxide staining or weathered accessory minerals
Category: igneous
Identified: August 7, 2026
Method: Standard

AI Estimated Value

Typically $0–$10 as a common unpolished cobble; perhaps $10–$30 if unusually attractive, polished, or provenance-labeled. It has no gemstone-grade value based on the photograph alone.

Not an official valuation

AI estimate for general interest only — not an appraisal. Actual value depends on condition, provenance, and market demand. Verify with a professional before buying, selling, or insuring.

This identification was generated by AI and may contain errors. Verify important details before relying on them — and never use an AI result to judge whether a specimen is safe to handle. Read the full disclaimer

Physical properties

Coarse, speckled crystalline texture with white to cream feldspar/quartz, dark gray-black minerals, and reddish-brown staining. Likely Mohs hardness about 6–7 overall; quartz is 7 and feldspar about 6. Luster is mostly vitreous on quartz and dull to pearly on weathered feldspar. No obvious cleavage is visible at the specimen scale, although feldspar grains have cleavage. Specific gravity typically about 2.6–2.7. The rounded shape indicates natural water or sediment abrasion, or prolonged weathering.

Formation & geological history

Most consistent with an intrusive felsic igneous rock that crystallized slowly from silica-rich magma deep underground, probably during a Precambrian to Phanerozoic intrusive event; its exact age cannot be determined from the photograph. Subsequent uplift, erosion, and transport rounded the cobble. Brown-red patches are likely oxidation of iron-bearing minerals or iron-rich alteration zones.

Uses & applications

Granite is widely used as crushed stone, aggregate, building stone, paving, monuments, countertops, and decorative material. This rounded specimen has little industrial value but may be kept as a field specimen, teaching sample, or lapidary material if it polishes attractively.

Geological facts

The image does not show enough detail for a definitive identification or precise geological age. The rock could also be a weathered granitic gneiss, arkosic conglomerate clast, or feldspar-rich pegmatitic rock. A fresh broken surface, hand lens, streak testing of the red material, and inspection for visible quartz/feldspar grain boundaries would improve confidence.

Field identification & locations

Common in continental crust and abundant in regions with exposed Precambrian shields, mountain belts, and granitic plutons. In the field, look for interlocking grains rather than sedimentary layers, abundant pale feldspar and glassy quartz, and a hardness sufficient to scratch glass. A magnet test may detect minor magnetite, while dilute acid normally produces no reaction unless carbonate veins are present. The rounded cobble may have come from a stream, beach, or glacial deposit.