Collection: Orbicular diorite

Orbicular diorite is a rare and very distinctive igneous rock, known for its spectacular appearance and concentric structures called "orbicles" .

🔹 What is orbicular diorite?

Orbicular diorite is a textural variant of diorite, a plutonic (deep-formed) rock composed primarily of plagioclase (calcium-sodium feldspar), amphibole (often hornblende), and sometimes biotite or pyroxene.

What makes orbicular diorite unique are the orbicules , small spherical or ellipsoidal structures (a few mm to several cm), made up of concentric layers of alternating minerals.

🔹 Formation of the orbicules

The formation of orbicules is still a subject of research, but several hypotheses exist:

• Rhythmic crystallization : orbicules would form around small nuclei (crystals, rock fragments, etc.) by successive precipitation of mineral layers from a magma in the process of crystallizing.

• Rapid chemical or thermal changes in magma, creating instabilities that promote layered growth.

• Interaction with hydrothermal fluids that could promote precipitation in concentric layers.

🔹 Visual characteristics

• General color : gray, black, white, sometimes tinged with green.

• Orbicles : Often dark in the center and surrounded by alternating light and dark layers.

• The appearance is often aesthetic and decorative , which makes this rock popular in lithotherapy, decoration or for art objects.

🔹 Where is orbicular diorite found?

It is extremely rare. Some famous deposits are known:

• Corsica (France) : in Saint-Jean-des-Balmes, Corsica, an outcrop of orbicular diorite is well documented.

• Sweden : in Ängsö.

• South Africa , Australia and a few other specific locations.

🔹 Use

• Highly prized for decoration , sculpture and as a geological collector's item .

• Some specimens are displayed in geology museums because of their rarity.