Topaz crystal structure, illustration. Topaz is a silicate mineral that includes Al(3+) and F(?: Al2SiO4F2. The tetrahedral silicate anion, SiO4(4? is shown in blue. The four silicate oxygen atoms are shared with aluminium, in an octahedral environment. Each aluminium atom is bonded to four oxygen atoms (dark orange) and two fluoride atoms (pink), which occupy a shared edge between two octahedra. Most forms of topaz gems are orange or yellow-brown.