I've used many versions of horn over the years, as knife scales and pen blanks. It's all prone to crack, and will often turn white if crushed. I never put a surface finish like CA on horn. Buff it to a high shine and coat liberally with wax to try to postpone the cracks as long as possible. Storing in a consistent temperature and humidity environment away from sunlight helps too.
And when it does crack anyway, in spite of everything you do, use some black ink to tint the crack, and some med CA to fill it, and then sand back down to the bare horn to repolish and buff.
Beautiful stuff, but a pain to work with.
