I have quite a number of victorian "Jet"style necklaces with black elements that are in fact stained horn. The advantages of horn were that it was not hard to obtain, it can be shaped not only by carving which takes a longish time but as it softens with heat it can be manipulated into shape such as the rings or chain links with only one cut, it can be pressed when heated to take relief designs, and it can be polished shiny. I also notice on the pieces I have that there is a certain translucency in some places -rather dark but brownish (like old sepia photographs).
One necklace I have (shown on p 25 of the Bead Jewellery Book lower right with ribbon ties) is made of pressed horn units, vulcanite links AND "bog oak"beads!