Diamond by H.Q. Frost

An unambitious girl from the Bronx has her bloodline to thank for an experience of a lifetime. The Reifs and Carbons are destined to be together, or so the Carbon family contract states.
Mag Reif volunteers over her younger, uninterested sister to marry the Duke, Callum Carbon, never expecting the arranged marriage to be as barbaric as the concept of the contract.
The Duke has no interest in being forced into marrying and impregnating the obnoxious diamond from the Bronx, and he makes sure she knows it. There’s a light at the end of the tunnel for both of them, just as soon as she bears a male heir. She can return to America, leaving behind her son of course, but it’ll get her away from the graphite Duke’s monstrous persona.
The Carbon Couple glimmer like crystals in public, but despite the lust, their fiery hate is turning them to coal.