The edges are easily damaged (pack singly- care needed during cutting).
Relatively high content of radioactive elements (uranium, thorium) cause large variations of physical properties.
Zircons with the highest values in optical properties and specific gravity are scientifically designed as normal or high zircons. Those with lower values are low zircons.
The alternation caused by radioactive elements in green zircons is also advanced and these stones can be nearly amorphous.
Hyacinth: yellow-red to red-brown variety
Starlite: blue variety (heat-treated)
In nature the gray-brown and red-brown zircons are the most common.
Colorless specimens are rare. Brown varieties are heat treated producing colorless or blue zircons.
These colors do not necessarily remain constant, ultra-violet rays or sunlight can produce changes.
Green zircons are rare in the trade, and are in demand by collectors.
Colorless heat-treated zircon has fraudulently offered for diamond as “matara-diamond”.
All zircons have strong double refraction, which can readily be seen under the loupe at the facet edges.
An important identification mark.

*Info by Walter Schumann “Gemstones of the World”