Bornit

Bornit, also known as bornite, is a copper-iron sulfide mineral with the chemical formula Cu5FeS4. It is an important copper ore mineral and commonly occurs with other sulfides such as chalcopyrite, pyrite, and sphalerite in hydrothermal veins and porphyry copper deposits. Bornit is widely recognized for its iridescent tarnish, which can display a rainbow of colors—purple, blue, yellow, and green—on fresh surfaces that oxidize in air.

In terms of physical properties, bornit crystallizes in the tetragonal system. It typically forms massive to granular aggregates and has a metallic luster. The color of a fresh surface ranges from gray to copper-brown, but tarnished specimens show vivid iridescence. The streak is gray to black. Mohs hardness is commonly cited as 3–4, and the specific gravity is about 4.9–5.1. It is relatively soft and tends to fracture rather than cleave.

Geologically, bornit forms at moderate to low temperatures in hydrothermal environments and plays a role in copper mineralization within porphyry systems. It can indicate copper-rich ore zones and is often found with chalcopyrite and other sulfides. The copper content in bornit is high, making it economically significant in suitable deposits. The mineral was named after the 18th‑century Austrian mineralogist Ignaz von Born. In the jewelry and gemstone trade, polished specimens are marketed as “peacock ore” because of their colorful tarnish.

Bornit is found in numerous localities worldwide, including North and South America, Europe, and Asia. It remains a common subject of mineral collections and educational demonstrations due to its striking appearance and copper-bearing nature.