[Greek dia, through.]
Most words beginning in dia- are based directly on Greek terms that already contain it. Some examples are diagnosis (Greek diagignōskein, distinguish or discern, from gignōskein, recognize or know), the identification of the nature of an illness by examining symptoms; diagonal (Greek gōnia, angle), joining two opposite corners; diagram (Greek diagraphein, mark out by lines, from graphein, write); and dialogue (Greek dialegesthai, converse with, from legein, speak). Diabolic is a less obvious example, as it derives ultimately from Greek diabolos, slanderer, from ballein, to throw (the name of a toy, diabolo, is a close relative via Latin and Italian).
A smaller proportion of words can be said unequivocally to have been generated using dia- as an English word-forming element. Among them are dielectric, having the property of transmitting electric force without conduction; diathermy (Greek thermon, heat), a therapeutic technique involving heating part of the body by high-frequency electric currents; diamagnetic, of a substance or body that tends to become magnetized in the opposite direction to the applied magnetic field; and epidiascope (Greek epi, upon, plus skopein, look at), an optical projector capable of giving images of both opaque and transparent objects.
Diamond was originally adamans in Latin (the source also of our adamant) but was altered in late Latin under the influence of Greek words in dia-.