For much of the nineteenth century, tortoiseshell was a luxury material that commanded high prices, whereas horn was a readily available material and inexpensive by comparison. By around 1830, the horn craftsmen found a method of clarifying and staining horn in imitation of tortoiseshell and, over succeeding decades, made combs, hairpins and other small items such as snuff boxes, fans and brooches. Being plausible reproductions of the real shell they, too, achieved similarly high prices and to all but the discerning eye, found a ready market until the advent of celluloid simulations in the 1880s.
The two combs pictured illustrate the remarkable similarities of each material, polished horn and lustrous tortoiseshell.
These combs are British, c. 1850-1870, and can be found on page 43 of The Comb: Its History and Development by Jen Cruse
Very interesting comparison! I note that the work on the tortoiseshell comb is more refined and elegant than on the horn one. But both are lovely and highly collectible.