Rutile

Rutile is a polymorph of titanium dioxide (TiO₂)
It may be of peculiar interest because:

• it contains the element titanium (Ti) whitch is one of the more abundant elements in earth crust.
• it has a reasonably high hardness (mohs 6.0 to 6.5)
• it features a reasonably simple (tetragonal) crystal lattice (in fact it's the defining minearal for the rutile structure)
• with the rutile structure it features the exact same structure as stishovite (a peculiarly interesting SiO₂ polymorph) but given it occurs naturally in high quantities (unlike stishovite) it likely has a higher thermodynamic stability. That is: it's less prone to diffusion into a more stable polymorph at higher temperatures.

Given both rutile and stishovite feature the same crystal structure it may be possible to mechanosynthesize checkerboard neo-polymorphic transitions by replacing some Ti with with Si in a regular pattern.

Other polymorphs of (TiO₂)

Another polymorph of (TiO₂) is anatase (mohs 5.5 to 6.0).
It also has a simple tetragonal crystal lattice but different from the rutile structure in that the unit cell is a bit bigger.

A third polymorph of (TiO₂) is brookite (mohs 5.5 to 6.0).
It also has a bigger unit cell than rutile and has the lower orthorombic crystal structure symmetry which may make it a bit less interesting as a potential base material.

Misc

• Titanium is supposedly especially abundant on the moon so future gem-gum products on the moon may feature a lot of rutile for structural parts.
• All (TiO₂) polymorphs have a high refractive index

External links

• https://en.wikipedia.org/wiki/Rutile (tetragonal | mohs 5.0 to 6.5 | 4.23 g/ccm)
• https://en.wikipedia.org/wiki/Anatase (tetragonal | mohs 5.5 to 6.0 | 3.79 – 3.97 g/ccm)
• https://en.wikipedia.org/wiki/Brookite (orthorhombic | mohs 5.5 to 6.0 | 4.133 g/ccm )