Information on shell cameos. Although occasionally used in Roman cameos, the earliest prevalent use of shell for cameo carving was during the Renaissance, in the 15th and 16th centuries. Before that time, cameos were carved from hardstone. The Renaissance cameos are typically white on a grayish background and were carved from the shell of a mussel or cowry, the latter a tropical mollusk.
In the mid 18th century, explorations revealed new shell varieties. Helmet shells (Cassis tuberosa) from the West Indies, and queen conch shells (Eustrombus gigas) from the Bahamas and West Indies, arrived in Europe. This sparked a big increase in the number of cameos that were carved from shells. Conch shells carve very well, but their color fades over time.
After 1850 demand for cameos grew, as they became popular souvenirs of the Grand Tour among the middle class. Reference: Wikipedia
Shell cameos are much easier to cut than those made from gemstones. Also, the raw material is cheaper and easier to acquire. They were popular in the 16th century and then again in the 18th and 19th centuries, when the passion for carved gemstones led to a parallel explosion in the market for cheaper shell cameos. They are still popular today. Reference: Victoria & Albert Museum