When a mineral is entirely devoid of crystalline structure, both externally and internally, it is said to be amorphous.
Perfect and distinct crystals are the rare exception, most mineral specimens being simply aggregates of imperfect crystals. In such cases, and when the mineral is amorphous, the structure of the mass may be:—
Columnar or fibrous.
Lamellar, foliaceous, or micaceous.
Granular.—When the grains or crystalline particles are invisible to the naked eye the mineral is called impalpable, compact, or massive.
And the external form of the mass may be:—
Botryoidal, having grape-like surfaces.
Stalactitic, forming stalactites or pendant columns.
Amygdaloidal or Concretionary, forming separate globular masses in the enclosing rock.
Dendritic, branching or arborescent.