Spinels can be found in some aluminous, Si-poor, metapelitic rocks, associated with other aluminous minerals. They occur in metacarbonate rocks, often associated with other Mg silicates. They are also found in some mafic and ultramafic igneous rocks, typically with olivine or pyroxenes.
Spinel with Pyroxenes in a Peridotite Xenolith from Kilbourne Hole, New Mexico
These photos show a high-relief green spinel grain in the upper right (PP). Most of the rest of the view is clinopyroxene (augite). A large grain of orthopyroxene, with slightly greater relief than the clinopyroxene, is on the left edge of the photos. Note that the spinel (isotropic) appears black in the XP view.
The field of view is 4.5 mm.