Hardness: 8 Mohs scale.
Specific gravity: between 3,58 y 4,06.
Optical properties: Isotropic.
Refractive index:: between 1,712 y 1,720. Gahnoespinelas can reach untill 1,753.
Color: Colorless (very rare), red in different blends, purple to violet, blue, blue-greenish, brownish-grey - black almost opaque (Ceylanite)Greynish-blue to dark blue (Gahnoespinel).
Over centuries it was mistaken with red ruby, nowadays even red spinel is rarest than ruby in same quality conditions ruby have higher prices than red spinel. Due to this confusion with rubies, over years two of the Jewels of the British were believed as rubies (Timur Ruby and Black Prince Ruby) when it really were red spinels.