“…Smith (e.g., in his 10th edition, 1949) said that yellow sapphire lost its color when heated and that radium (i.e., irradiation) either deepened the color or developed it in a colorless stone. More recent studies have expanded our knowledge of this behavior (Crowningshield and Nassau, 1981;Fryer et al, 1983aFryer et al, , 1983bKoivula, 1986;Nassau, 1980Nassau, , 198 1, 1984Schiffmann, 1981;Schmetzer and Bank, 1980;Schmetzer et al, 1983). Some studies (e.g., Schmetzer et al, 1983) have suggested that the yellow color may arise from color centers, or iron, or both.…”