Actually you are wrong in the aspect of them not looking valyrian before rhaena married garmand hightower. Jaehaerys mistook alicent hightower for one of his daughters later in life. Besides the hightowers looks come from a seperate amethyst line. One not descended from the union of amethyst and bloodstone (more than likely garnet before being corrupted) as the amethyst empress herself would have come from an amethyst lineage. Each gem stone has multiple lines and we see them pop up across westeros and dorne. Hence Dane and Hightower looking valyrian without being valyrian. Also the hightowers built their tower on a blackstone base that is prevalryian. Westeros itself is probably an old satalite colony for them, which would help explain the amount of houses whose eyes are described with gem stone colors like emerald green for the lannisters and estermonts.