Usually in gemmy red-brown to green crystals to several mm in vugs. Crystals may be individual complexly modified tetrahedra, but rounded crystal forms are common. Clusters of crystals may be composed of a few individuals or may show only the projecting apices of tetrahedra and in extreme cases nearly smooth spherical clusters. Also commonly occurring as globular aggregates.
Synthetic material doped with REE has technically interesting luminescence properties.