Teineite is a tellurite mineral with the formula Cu(TeO3).

It has a Mohs hardness of 2.5 and it comes in many different shades of blue, ranging from cerulean blue to bluish-gray.

The mineral millsite has the same chemical composition, but crystallizes in the monoclinic system, while teineite crystallizes in the orthorhombic system.

[2][3][4] Teineite was first identified in the Teine mine, Sapporo, Hokkaido, Japan, where the name of this mineral originates.

It occurs in veins where copper- and tellurium-bearing sulfides were oxidized and is often associated with tellurite, tellurium, pyrite, tetrahedrite, sphalerite, azurite, malachite, quartz, baryte, hessite, galena, bornite, cerussite, chlorargyrite, quetzalcoatlite, cuprite and graemite.