Rhombohedral

It forms four major polymorphs : α-rhombohedral and β-rhombohedral (α-R and β-R), γ and β-tetragonal (β-T); α-tetragonal phase also exists (α-T), but is very difficult to produce without significant contamination.

Acute scalenohedral crystals are sometimes referred to as "dogtooth spar" while the rhombohedral form is sometimes referred to as "nailhead spar".

At ambient conditions, samarium normally assumes a rhombohedral structure (α form).

Carbonates tend to be brittle, many have rhombohedral cleavage, and all react with acid.

In monoclinic, rhombohedral, tetragonal, and trigonal/hexagonal systems there is one unique axis (sometimes called the principal axis) which has higher rotational symmetry than the other two axes.

Polonium has two allotropes, which are known as α-polonium and β-polonium. citation α-polonium has a cubic crystal structure and converts the rhombohedral β-polonium at 36 °C.

Reddish rhombohedral calcite crystals from China.