* Color: Black or dark brown. Ivory is a pale yellowish-white color, so the opposite would be a very dark color.
* Material: Ebony. Ivory is a material derived from elephant tusks. Ebony is a hard, dark wood often used as a contrasting material to ivory.
* Symbolism: Honesty/Transparency. Ivory is often associated with luxury, purity, and even power. The opposite could be materials associated with honesty, transparency, and down-to-earthness, like wood, stone, or even metal.
Ultimately, the "opposite" of ivory depends on the context of your question.