Not too sure how to post a link between forums - this is what Kiwipaul says in his article about these hard stone cameos...
"Antique Pearl Jewelry that YOU Could Own - The Victorian Period
Black materials like agate, jet and onyx were often used in jewelry of this period, and contrasting white on black hardstone cameos were popular. The wealthy would have a custom cameo carved to depict a loved one, or for a little less one could buy an off-the-shelf cameo portrait of an admired celebrity or historical character.
Pearl borders provided an attractive decorative detail for black and white jewelry, seen here in this fine Mid-Victorian hardstone cameo pendant. The subject is Mary Queen of Scots, identified by her necklace of pearls. "