Named for its African country of origin, tanzanite is a blue stone used as a substitute for sapphire. Unlike the sapphire, tanzanites are a three-coloured gemstone. They contain three different shades of reds, purples, and blues that are visible to the naked eye. Synthetic forsterite and blue-violet glass are both sold as tanzanite imitations. Besides inspection from a jeweller, there are steps you can take to determine the authenticity of your tanzanite.
Place stone on white surface. If the stone is mounted, place light below.
Hold filter around 36 cm (14 inches) from your eye, right over the stone.
Move filter from side to side, using the strip as a guide. View the stone from several different angles while moving the filter to ensure accurate results.
Compare colours seen with enclosed instruction chart. Imitation tanzanite such as fosterite will show a green colour through the filter. Authentic tanzanite will appear a pinkish-orange colour through the filter.
There are test kits that include both a filter and dichroscope, many also have a detailed chart with colour descriptions.
Filters may also be called "Hanneman" filter.
Using fluorescent or sunlight will cause incorrect readings with a filter.
Stones that have been set with a dark solid backing may be harder to test using a dichroscope or filter.