Hardness rhymes with:
weirdness, awkwardness, backwardness, badness, baldness, blandness, blessedness, blindness, bloatedness, boldness, coldness, crookedness, discontentedness, fadness, farsightedness, fondness, goodness, handedness, high-spiritedness, kindness, lightheadedness, low-spiritedness, madness, mindedness, nearsightedness, poor-spiritedness, preparedness, public-spiritedness, redness, rudeness, sadness, sandness, shrewdness, soundness, spiritedness, vividness, weirdness, wickedness, wildness
Hardness sounds like:
harden's, hardening, hardens, harding, harding's, harting, hartness, hartong, hartung, heartening, herding, herting, hoarding, hurting