ZrSiO 4 Composition Zirconium silicate, often with some hafnium and occasionally with some uranium, thorium, and yttrium. When impure can be as low as 6 . Most often as short, stubby , prismatic crystals, which are almost always terminated . Crystals also occur in elongated , terminated prisms. Doubly terminated crystals are not uncommon.
Radioactive zircon is characterized by rounded crystal faces.
Zircon also occurs as grains , as fibrous aggregates, and as rounded, waterworn pebbles. Twinned Zircon crystals are uncommon. Transparency Transparent to opaque Specific Gravity 4.6 - 4.8 Luster Greasy to adamantine. Radioactive Zircon has a pitchy luster. This unstable form of Zircon, called Cyrtolite , is characterized by rounded, almost domal crystals, which are usually dull or pitchy in luster .