What is the adjective of fascinate?

The Word 'fascinating' is the present participle, present tense of the verb to fascinate.

The present participle of the verb also functions as an adjective and a gerund, a verbal noun.

The gerund 'fascinating' is an abstract noun as a Word for a concept.

Examples:

The new microscope is fascinating the second graders. (verb)

That is a fascinating idea. (adjective)

The author has a knack for fascinating her readers. (noun, object of the preposition 'for')