The quality or state of being gravelly; the texture or consistency of gravel.
From gravelly + -ness (suffix forming abstract nouns). This creates an abstract noun describing the property of having a gravelly character.
Adding -ness to adjectives to make nouns is so common we barely notice it, but it's actually a super efficient way English creates whole new concept words—gravelliness is the essential 'gravel-ness' of something.
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