You aren't setting a BlockRenderLayer in BlockConjuringAltar so it doesn't render properly in the world.
It would need to be CUTOUT or CUTOUT_MIPPED for it to render as intended.

Mod version: 0.2.0
Forge version: 1.12-14.21.1.2443

That's weird as I never got this issue myself, and that's weird how the altars in your screenshots have diamonds as it's not supposed to have any.

Anyway, implemented the fix with 90189da as I don't see any harm with it, may wait for a bit as I'm preparing a bigger update.

Oh! I found the issue on my end too. I was using this resource pack here which replaces the existing enchanting table model (which you're using as your altar's model parent), and the layer it was adding needed to be on the cutout layer, hence the whiteness on my screenshot. The easiest solution to avoid conflicts with other mods/resource packs would be to have your own altar model (just copy the vanilla enchanting table model), otherwise your model may break/look odd when a resource pack changes the enchanting table model.