How to Disclose Your Relationship Status, 1890

 'If a gentleman wants a wife, he wears a ring on the first finger of the left hand; if he is engaged, he wears it on the second finger; if married on the third; and on the fourth if he never intends to be married. When a lady is not engaged, she wears a hoop or diamond on her first finger; if engaged, on the second; if married, on the third; and on the fourth if she intends to die unmarried.'

The Mystery of Love, Courtship, and Marriage Explained

And a diamond thumb ring is universal language for 'it's complicated.'