Used attributively to describe tools, stones, etc., related to the act or purpose of sharpening.
Used as an attributive noun to modify another noun (e.g., "sharpening kit")
sharpeninggerund / present participle Used attributively: e.g., "a sharpening stone"
sharpengerund / present participle ‘sharpening’ is used attributively (e.g., “a sharpening stone”) and thus functions as an adjective.