The Samarian Spinel is the largest known spinel in the world, weighing . Taaffeite occurs in carbonate rocks alongside fluorite, mica, spinel and tourmaline. Distinctive accessory minerals include&colon; fluorite, topaz, apatite, spinel, allanite, sphene and cassiterite. Associated minerals include vesuvianite, chlorite, epidote, spinel, calcite, dolomite and magnetite. Aluminium also occurs in the minerals beryl, cryolite, garnet, spinel, and turquoise. Pleonaste (black spinel) and black diamond are examples of opaque faceted gemstones. Peridotite (primarily spinel harzburgite) minerals undergo serpentinization and form magnetite and serpentine minerals. ORNL devised a cathode made up of spinel manganese oxide further reducing corrosion. Other gems also occur in colour-change varieties, including (but not limited to) sapphire, garnet, spinel. It can also occur in garnet, diopside and spinel. Plagioclase, quartz, gypsum, and spinel often exhibit contact twinning. It may also be substituted by spinel or sodalite, or by dyed jasper or howlite. The pure blue coloration of spinel is caused by small additions of cobalt. Xenoliths of dunite, peridotite and spinel lherzolite in basaltic lava flows are one example.