Polymorphs of silicon dioxide

More commonly known SiO₂ gemstones

• Common Quartz
• amorphous forms (properties similar to: chalcedon, opal, obsidian, ...)
  mechanosynthesis would allow for synthesis of pseudo amorphous configurations
  Neo-polymorphs

Especially interesting since unusually hard and dense

• Stishovite Mohs 8.5-9.0 tetragonal –– may be transitionable into other gemstomes with the same rutile structure forming pseudo phase diagrams
• Seifertite Mohs ~9 orthorhombic –– (scrutinyte structure)

Both are metastable very hard and very dense.
They may not be very resilient against high temperatures.

Other polymorphs of SiO₂

• (wiki-TODO: add some more known ones)
• https://en.wikipedia.org/wiki/Category:Silica_polymorphs

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Basic low density polymprphs:

• https://en.wikipedia.org/wiki/Keatite Mohs 5.75 tetragonal
• https://en.wikipedia.org/wiki/Tridymite Mohs 7 orthorhombic (high temp modification)
• https://en.wikipedia.org/wiki/Cristobalite Mohs 6-7 tetragonal
  (& high temp β-cristobalite cubic - diamond like structure)
• https://en.wikipedia.org/wiki/Melanophlogite Mohs 6.5-7.0 cubic or tetragonal
• https://en.wikipedia.org/wiki/Coesite Mohs 7.7-8.0 monoclinic

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Exotic stuff:

• https://en.wikipedia.org/wiki/Chibaite Mohs 7 cubic structure chalrate:
  
• https://en.wikipedia.org/wiki/2D_silica
• CaCl₂-type and α-PbO₂-type (high pressure phases)
  2003 – Stability of CaCl₂-type and α-PbO2-type SiO₂ at high pressure and temperature determined by in-situ X-ray measurements

Related

• Silicon, Oxygen
• Gemstone like compound
• Neo-polymorphs