What does amorphous mean in mineralogy ?

Amorphous : definition

This term applies to non-crystalline mineral substances, that is to say having no ordered atomic structure (no crystal lattice), and not diffracting with X-rays (opal, volcanic glass). Some of these substances, like opals, are however classified as minerals.

Attention, amorphous does not mean that the mineral does not have no faces expressed. It is the xenomorphic word which must then be used in this case.