Epididymite's structure is orthorhombic and eudidymite's structure is monoclinic in symmetry . The best known dimorphs are diamond and graphite , both of which are made of carbon, C , but have very different structures. Eudidymite and epididymite are found in the unusual igneous intrusive rocks called agpaites. These rocks are similar to nepheline-syenites but are enriched in alkali metals and exotic elements and conversely low in aluminum and silica. Beryllium is one of those exotic elements that is in unusual concentrations in agpaites and thus allows minerals like eudidymite to form.
Color is usually white or colorless, but can be pale pink, gray, yellow, blue or violet.
Luster is vitreous to silky or pearly. Crystals are transparent to translucent. Crystal Habits include tabular crystals and aggregate spherulites. Twinning forms stellate figures with wedge shaped spokes. Fracture is uneven.