There was a question of the "grey eyed" epithet of Athena on Mythology recently, and one of the answers related to Athena's perceptiveness.
(The choice of "grey eyed" by translators is both poetic, and relates to a proposed connection between the Greek word of own and an adjective used for the color of the sea.)
What interests me here is the quality of perceptiveness, which enhances information gathering and analysis, leading to Athena's superior judgement and wisdom.
How is "grey eyed" related to "steely eyed" beyond mere color? What are early appearances of "steely eyed"?
Note: I don't agree with the common definition of steely eyed, which seems to miss the point of the idiom. My understanding of steely eyed, based on context and usage, is related to realism, rationality, and canniness drawn from experience. Toughness/determination is merely the affect or appearance, and doesn't get to the root of the usage. That said, toughness and determination are traits also possessed by the armored, steely-eyed Athena. Where the common definition intersects with the deeper meaning is in regards to "mindset" (The mind is also the domain of Athena.) The determination connoted by being steely-eyed comes from a realistic assessment of a given situation, which is to say, analysis.
Part of what leads me to this conclusion is that, in this case, the adjective is applied very specifically to an organ of sense, which is to say perception. This is quite distinct from merely being determined as in having a "grim visage" or other idioms, and distinct even from "having a thick skin", which is also sensory.
To further this point, steel can be keen (have a keen edge) and it is notable that definitions of keen in include "sharp or penetrating", where sharp is also a term for mental acumen, and penetrating can be applied in the context of perception. (In Hermeticism, the sword is a symbol of intellect.) Even the use of keen for enthusiasm requires intellectual engagement.