The skeletons of mammals originate as soft cartilage that gradually transforms into hard bone (in humans, the process begins in the womb and continues until late adolescence). This bone-building process has been called ossification since the late 17th century, and the verb ossify arrived soon after the noun. Both terms have come to refer to figurative types of hardening, such as that of the heart, mind, or soul. The words come from the Latin root os, meaning "bone." Os has also entered English as a synonym of bone in scientific contexts.

What is the difference between ossify and calcify?

Medically speaking, ossify refers to the process by which bone forms, or by which tissue (usually cartilage) changes into bone. Ossification is a natural process that starts in utero and which comprises several different steps—one of which is the deposit of calcium salts, also known as calcification. Calcify, however, only refers to the deposit of calcium salts in soft tissue and is not synonymous with ossify. Ossification creates bone tissue, which is more than simply a deposit of calcium salts.

Both ossify and calcify have gained more general uses as well. Calcify refers to hardening, to becoming inflexible and unable to change:

What were once upstart revisionist currents calcified into self-regarding academic sub-specialties, sponsoring plenty of analysis but little fundamental debate.
— Sean Wilentz, The New Republic, 2 July 2001

Ossify refers to becoming inflexible, conventional, and resistant to change:

For these writers, the ossified ideologies of the world, imbedded in the communal imagination, block vision, and as artists they respond not by criticism from without but by confrontation from within.
—Robert Coover, The New York Times Book Review, 18 Mar. 1984

While ossify generally has a slightly more disparaging connotation to it than calcify does in general uses, our evidence shows that the two words are beginning to merge semantically.