Silicon

Silicon dioxide SiO₂

Beside the commonly known quartz and amorphous glass there are many polymorphs of silicon dioxide (tectosilicates).

• Quartz (Mohs 7 by definition | density 2.65 g/cm³)
  α-quartz: trigonal -- β-quartz: hexagonal

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• Stishovite (Mohs 9-9.5 | density 4.287 g/cm³ | tetragonal - resembles rutile TiO₂)
• Seifertite (Mohs ~9 | density 4.294 g/cm³ | orthorhombic - resembles scrutinyite α-PbO₂)

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• Coesite (Mohs 7.5 | density 2.92 g/cm³ (calculated) | monoclinic)
• Cristobalite (Mohs 6-7 | density 2.32–2.36 g/cm³)
  α-Cristobalite tetragonal
  β-Cristobalite cubic - resembles diamond / ZnS

• Tridymite (Mohs 7 | density 2.25–2.28 g/cm³) there are seven crystal phases of tridymite
  two hexagonal: HP (β) & LHP -- three orthorhombic: OC (α) & OS & OP -- two monoclinic: MC & MX
• Moganite (not MoRganite!) (Mohs 6 | 2.52-2.58 g/cm³ | monoclinic)

Related: Binary diamondoid compound