Salts of oxoacids

The X-O-X bonds present in those compounds increase the space between the spacially linking X atoms. This lead to a lower density of bonds in cross sections inclusion of bigger voids thus higher porousity. Due to the porousness of these compounds it is harder to get the surfaces flat - figuratively like the surface of a pumice stone. It's impossible to get them as smooth as passivated diamond. [todo: investigate wheter supralubricating bearings can be constructed from these types of diamondoid compounds]

Quartz

Not exactly a salt but related ...

Phosphate minerals

(wikipedia)

• Especially interesting: Hydoxy- Fluor- & Clorappatite Ca₅(PO₄)₃(F,Cl,OH) - (Mohs 5) - a biomineral
• Lazulite (wikipedia) & Scorzalite (wikipedia) magnesium iron aluminium phosphates - (Mohs 5.5-6)
• Phosphophyllite (wikipedia) - zinc iron phosphate - rather soft (Mohs 3.5)
• Pb₅(PO₄)₃Cl pyromorphite (wikipedia) - relatively hard for a lead mineral (Mohs 3.5)
• YPO₄ Xenotime (wikipedia) - rare yttrium - high symmetry (tetragonal) - (Mohs 4.5)

Sulfate minerals

(wikipedia)

Sulfate minerals are generally rather soft with few exceptions.
One of the harder ones is brochantite (wikipedia) - (Mohs 3.5-4)