Ebonite, hard rubber, whatever you want to call it, was the first material used for fountain pens. It is light and warm to the touch making it comfortable in the hand. Though none of the larger modern companies use hard rubber much if at all, Indian manufacturers use it creatively.
It is said by some that the main reason celluloid became so popular is because hard rubber is brittle. I don’t find that to be true. Red hard rubber and the various varieties of mottled hard rubber are more brittle than black hard rubber, but I’m not sure that any of them are more brittle than celluloid itself. Where the material is thin, as in the cap lip, hard rubber, celluloid or the plastic that Aerometric Parker Duofolds were made from, all have a tendency to crack. Those pens that have a tendency to mysteriously crack in thicker, unstressed areas are often celluloid.
No, I think the real reason celluloid caught on right away was because it could be made in so many colours, and with such depth of colour. Though some highly decorative pens were made in hard rubber like the Wahl Eversharp wood grains and the various Waterman ripples, there was a limit to the colours and patterns that could be produced. They tended to lose their shine quickly, and with the shine went the little depth they had once had. Celluloid is harder and retains its shine well. Chasing in celluloid does not wear as quickly as it does in hard rubber.
All the major companies made pens in black hard rubber and all faded quite quickly. The mystery is that there is a group of 1920s British pens in black hard rubber that never fade. Was there a cheaper version of black hard rubber that only incidentally had the property of holding its colour and shine which only became clear years later?
In the US Waterman and Wahl Eversharp persisted with hard rubber long after Parker and Sheaffer moved to celluloid, but eventually even they made the switch. There was a period when no gold-nib pen maker used hard rubber. Celluloid, casein and some other, less definable plastics ruled the roost. During World War Two, Mabie Todd returned to black hard rubber. No one knows why. Did they have unused stocks of the material that reduced manufacturing costs? It was clearly the case that the colour black was very popular in 40s and 50s Britain, and perhaps that made the return to black hard rubber easier. In any case, many pens were made of that material and those that have not faded too much are handsome and comfortable pens.