Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate), caused by small amounts of vanadium.

[5] Tanzanite is noted for its remarkably strong trichroism, appearing alternately blue, violet and burgundy depending on crystal orientation.

The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination.

In its rough state tanzanite is coloured a reddish brown to clear, and it requires heat treatment to remove the brownish "veil" and bring out the blue violet of the stone.

[8] Tanzanite was formed around 585 million years ago during the mid-Ediacaran Period by massive plate tectonic activity and intense heat in the area that would later become Mount Kilimanjaro.

Shortly thereafter, the stones were shown to John Saul, a Nairobi-based consulting geologist and gemstone wholesaler who was then mining aquamarine in the region around Mount Kenya.

Hyman Saul took the samples to the Gemological Institute of America which correctly identified the new gem as a variety of the mineral zoisite.

[citation needed] In June 2003, the Tanzanian government introduced legislation banning the export of unprocessed tanzanite to India.

The reason for the ban is to attempt to spur development of local processing facilities, thereby boosting the economy and recouping profits.

[25] Block C, by far the largest site, has been estimated at 87,100,000 carats (17,400 kg; 38,400 lb) with a Life of Mine (LOM) expected to last until the 2040s.

[28] The new system's colour-grading scales divide tanzanite colors into a range of hues, between bluish-violet, indigo and violetish-blue.

As a result of this phenomenon, a multitude of colors have been observed in various specimens: shades of purple, violet, indigo, blue, cyan, green, yellow, orange, red and brown.

[29] Clarity grading in coloured gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20/20 vision).

Tanzanite forms as a brownish crystal and is trichroic, which means it shows three colours – brown, blue and violet – concurrently.

Heating, either underground naturally by metamorphic processes, or artificially, removes the brown or burgundy colour component to produce a stronger violet-blue color and makes the stone "dichroic", which means it only reflects blue and violet.

Pleochroism has a physical property in which the gemstone will appear to have multiple colours based on the angle of the light hitting the stone.