Likely French Artist: Adele H.

Cameo Portrait of a Mid-Nineteenth Century Gentleman,
Carved into Shell and Mounted in a 14 Karat Gold and
Black Enamel Brooch

Likely French
dated 1859
Signed and dated to the reverse, "Adele H., 1859"

1 x 1 1/4 inches (sight);
full brooch measures 1 1/2 x 1 3/4 inches

Not all antique portraits were taken with a camera or painted with a brush.
Many a Caesar had his countenance sculpted in marble and, centuries
later (especially popular in the 18th and 19th centuries), many a
wealthy individual had their portrait carved into stone or shell by
a master cameo cutter.

Similar objects in the Tormey-Holder Collection
(click photos for larger views and additional information):

Cameo Portrait of an Mid-Nineteenth Century Boy,
Carved into Shell and Mounted in a 15 Karat Gold Brooch

Cameo Portrait of an Early Twentieth Century Gentleman, Carved
into Striated Black Onyx and Mounted in an 18 Karat Gold Surround