Currently, the combiner allows the creation of every ore block, both those in vanilla and those we add, with the exception of coal, emerald, and nether quartz ore. Nether quartz ore is currently blocked on #4648, but there's nothing stopping the other two. I propose that we add these recipes. However, I'm not sure exactly what they should be, since we don't add emerald or coal dust. Here's some options I'm thinking:

1. Just use coal and emeralds directly instead of dusts in the combiner. There's already sort of precedent for this with the flint and lapis lazuli recipes.
2. Add coal and emerald dust (created by crushing coal and emeralds, not through the ore processing chain), and use these new dusts in the combiner recipes.
3. Add recipes from coal and emerald dusts to their ores, but don't actually add the dusts, instead relying on another mod for that.
4. Check at runtime whether any other mods add coal or emerald dust. If so, do method 3 for that ore; otherwise, do method 1.

Which option should we go with? (I plan to create a PR for this once I know which way to go with it.)

Just use the Coal/Emeralds directly (while making sure it's not possible to exploit it for infinite items of course)

+1 for what @thiakil said. Number 1 looks best to me ;)