Hardness was a measure of a material's resistance to certain types of deformation, such as indentation or abrasion. Inversely, softness was how easily those deformations could occur.
In 2266, Federation Starfleet officer Spock claimed that cast rodinium was the hardest substance known to their science. (TOS: "Balance of Terror")
In 2267, Captain James T. Kirk expressed uncertainty in diamonds being the hardest substance known in the universe. Spock later claimed that diamonds were the hardest known substance and thus would make formidable projectiles when impelled by the sufficient force of gunpowder. (TOS: "Arena")
In 2268, ermines, tribbles, and Regulan bloodworms were described as soft. (TOS: "The Trouble with Tribbles")
That same year, Kirk stated that pure tritanium was twenty times as hard as diamond, and Spock corrected him that it was exactly 21.4 times as hard. (TOS: "Obsession")