Members of the spinel mineral group occur in many igneous and metamorphic rocks, and  span a wide compositional range, including varieties such as magnetite and chromite that are opaque.  Translucent examples have high  positive relief (comparable to garnet), and are commonly shades of brown and green in plane light.  The pale pink spinel in A, enclosed in orthopyroxene, is near end-member MgAl₂O₄ in composition.  Spinels are isometric and, therefore, isotropic (B).  The brown variety picotite, shown in C interstitial to clinopyroxene is richer in iron and chromium.  In D, abundant euhedral opaque chromite grains are poikilitically enclosed by orthopyroxene.  A and B are from a spinel/garnet websterite xenolith from the Premier kimberlite, South Africa, C is from a spinel websterite from the Baldissero Massif, Italian Alps, and D is from the Muskox Intrusion, N.W.T.  A and B are 2.2 mm across, C and D are 5.5 mm across. A, C, D ppl, B x-nicols.