Most gem-grade zircons show a high degree of birefringence which, on stones cut with a table and pavilion cuts (i.e., nearly all cut stones), can be seen as the apparent doubling-up of the latter when viewed through the former, and this characteristic can be used to distinguish them from diamonds and cubic zirconias as well as soda-lime glass, none of which show this characteristic. However, some zircons from Sri Lanka display only weak or no birefringence at all, and some other Sri Lanka stones may show clear birefringence in one place and little or none in another part of the same cut stone. Other gemstones also display birefringence, so while the presence of this characteristic may help distinguish a given zircon from a diamond or a CZ, it will not help distinguish it from, for example, a topaz gemstone. The high specific gravity of zircon, however, can usually separate it from any other gem and is simple to test.