Final answer:
The actual moment Medusa's head turns to stone is not described in detail; the focus is on her transformation into a gorgon and Perseus's slaying of her. Perseus uses divine gifts to avoid her fatal gaze and decapitates her, after which her head retains its petrifying power but is controlled by Perseus.
Step-by-step explanation:
The moment when Medusa's head turns to stone is not described explicitly in the provided text; rather, the texts focus on the circumstances leading up to and following that pivotal moment. Initially, Medusa is transformed into a gorgon by Athena, which turns her into a creature with the deadly power to petrify anyone who gazes directly at her. This transformation is a punishment for Medusa's pride in comparing herself to a goddess.
When Perseus undertakes the task of slaying Medusa, he is equipped with divine tools, including a reflective shield from Athena and winged sandals from Hermes, enabling him to view Medusa's reflection and decapitate her without falling prey to her petrifying gaze. The description of Medusa's head after being severed indicates that it retains its power; a power effectively neutralized once in Perseus's possession as he uses it in further quests.
Artistic representations of this myth, such as Cellini's Perseus Holding the Head of Medusa, render the aftermath with Perseus holding the severed head, a powerful image illustrating the moment when the perilous gaze of Medusa is finally subdued.