32 Finally, art which is based upon Christian principles and is animated by their spirit as something faithfully interpreting the sincere and freely expressed devotion of the faithful, has since the Council of Ephesus portrayed Mary as Queen and Empress seated upon a royal throne adorned with royal insignia, crowned with the royal diadem and surrounded by the host of angels and saints in heaven, and ruling not only over nature and its powers but also over the machinations of Satan. Iconography, in representing the royal dignity of the Blessed Virgin Mary, has ever been enriched with works of highest artistic value and greatest beauty; it has even taken the form of representing colorfully the divine Redeemer crowning His mother with a resplendent diadem.
Source: Ad Caeli Reginam (Vatican.va)