What is "diascopy"

Diascopy is a test for blanchability performed by applying pressure with a finger or glass slide and observing color changes.

It is used to determine whether a lesion is vascular ( inflammatory or congenital), nonvascular ( nevus), or hemorrhagic ( petechia or purpura). Hemorrhagic lesions and nonvascular lesions do not blanch; inflammatory lesions do. Diascopy is sometimes used to identify sarcoid skin lesions, which, when tested, turn an apple jelly color.