Hardened, indurated, callous mean grown or become hard.
These terms are comparable in both literal (compare HARDEN 1 ) and extended use.
Hardened is the most inclusive because it is applicable to any substance, whether originally fluid or solid, or loose or firm in texture, or elastic or inelastic, that has become solider and firmer and increasingly resistant to efforts to cut, pierce, or bend.
Consequently, in extended use hardened usually implies a fixing or setting with loss of qualities (as flexibility, elasticity, pliancy, susceptibility, and impressionableness) indicative of a capacity for change.
Indurated is common in geological and in medical use with the implication of an increase of hardness or compactness usually in something already firm or hard and sometimes to the point of abnormality.
In its extended use it usually stresses abnormal or excessive hardness or stoniness that repels all efforts to penetrate or to soften.
Callous, in its earliest and still common sense, implies a hardening and thickening of the skin by constant pressure or friction and a consequent loss of sensibility in the part affected.
In extended use it usually also implies a loss of sensibility or an insensitiveness that results from constant experience, but sometimes callous means simply unfeeling and carries little implication of the process of hardening.