Tanzanite is a trichroic mineral, which means that a different colour may be observed along each of the three vibrational directions relating to the crystal symmetry. A polarised spectrum through the UV-Vis-NIR regions will trace the absorption of light along each of these vibrational directions and clearly identifies each of the pleochroic hues observed. The three colours are typically violetish-blue, purple and greenish-yellow.

An infrared spectrum is utilised to classify diamonds into four basic groups. Those diamonds which do not reveal absorption bands, between 500 and 1500 wave numbers, relating to nitrogen impurities, are classified as Type IIa. Only a very small percentage of diamonds are classified as Type IIa. However, a number of the world's largest "D/IF" diamonds are Type IIa and it is the relative lack of impurities that account for their superior colour.

Rubies from the deposits near the town of Mong Hsu in Burma, are typically recovered possessing a dark, violetish-blue core colour zone. Incorporating the use of heat treatment techniques, these colour zones may be removed. However, in some stones from this source, dense whitish clouds may precipitate during the process thereby reducing the transparency of the gemstone. The black line reveals the presence of minute particles of diaspore (AlOOH), included within the ruby host. After heat treatment, an alteration process has taken place, where the dehydroxylation of the diaspore particles has created structurally bonded OH groups, as illustrated by the yellow line.