Why are the Russian icons 'Mystic Marriage of Saint Catherine' considered so rare?

This is a rare icon called the 'Mystic Marriage of Saint Catherine'.

St. Catherine the Great Martyr is an early Christian saint who lived in Egypt in the 3rd century AD. Her mystical betrothal to Christ is a frequent subject in European fine art. In the paintings, an infant Jesus gives St. Catherine a wedding ring. This is a symbol of spiritual union (according to one version,  St. Catherine was a nun, that is, a "bride of Christ").

St. Catherine was venerated throughout the Christian world. Young girls asked her for a good groom and married people prayed to her to strengthen the marriage and family happiness. She is also considered the patroness of sciences, students, theologians and philosophers.

In Russian iconography, the plot with this mystical betrothal is extremely rare. And, so, this icon is a unique relic. The image in a silver frame once belonged to the Moscow Grenadier Regiment and was especially honored by officers. The icon also has a memorial inscription: In 1797, it was commissioned by Captain Gryazev, a participant of Suvorov's European campaigns.