This work may have been painted in Bologna after the artist fled from Rome. Saint Catherine is identified by the wheel upon which she was tortured before being miraculously saved. The figure in the foreground may be the hermit who converted her. After her conversion she dreamt that the infant Jesus placed a ring on her finger in a 'mystic marriage'. The ring with its blue stone is in the centre of the painting.
From The National Gallery Podcast: Episode Thirteen, November 2007