This is the heavy diamond armor, and it's a known issue. It happens because nobody's bothered to texture/model it yet. Most armor models are some degree of unfinished right now, but Heavy Armor definitely has it worst.

I had a feeling this was the case, as this is the alpha release, some things are bound to be unfinished :3

Btw, Inquisition armor isn't available in survival yet, but it's also missing its model.