Calcinosis cutis is a deposit of calcium, which is stony-hard, whereas a tophus is a deposit of softer, chalky monosodium urate crystals.
A cyst is a smooth, dome-shaped nodule containing keratin, and is less firm than a tophus and not typically located over a joint.
A keloid is a firm, rubbery scar that arises after skin trauma, whereas a tophus is a depositional mass related to a metabolic disorder.
A rheumatoid nodule is a firm, non-tender nodule found over pressure points in a patient with rheumatoid arthritis; it does not contain crystalline material like a tophus.
Xanthomas are yellowish, lipid-filled deposits associated with dyslipidemia, a different color and composition than the whitish, urate-filled tophi.
