igneous
Orbicular Granite (specifically Mustard or Yellow Tonalite/Granodiorite)
Orbicular Quartz Diorite / Tonalite
Hardness: 6-7 (Mohs scale), Color: Mustard yellow feldspar groundmass with black hornblende or biotite inclusions, Luster: Vitreous to dull, Structure: Phaneritic (coarse-grained) with orbicular/ellipsoidal clusters.
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Mustard yellow feldspar groundmass with black hornblende or biotite inclusions, Luster: Vitreous to dull, Structure: Phaneritic (coarse-grained) with orbicular/ellipsoidal clusters.
Formation & geological history
Formed from the slow cooling of silica-rich magma deep within the Earth's crust. The yellow coloration usually results from iron oxide staining or high concentrations of alkaline feldspar. Most specimens are from the Phanerozoic eon.
Uses & applications
Used primarily as decorative stone, lapidary material for polishing into cabochons or spheres, and as a ornamental building facade material.
Geological facts
Orbicular rocks are rare and occur when minerals crystallize in concentric layers around a nucleus while still in a liquid magma state. This specific yellow-orange variety is often nicknamed 'Gold Granite' or 'Mustard Jasper' by lapidary hobbyists, though it is geologically a granite.
Field identification & locations
Identify by the salt-and-pepper texture superimposed on an orange/yellow base. Commonly found in large batholiths in places like Western Australia, Brazil, and parts of the Southwestern United States. It handles a high polish well.
