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Global Institute of Gemology • Gemology Tools
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PLEOCHROISM / GEMSTONE COLOUR

Dichroscope

A dichroscope shows differences in colour absorption along distinct vibration directions, making pleochroism observable in many coloured, anisotropic minerals.

Dichroscope instrument, cut out from the original photographic collection

What pleochroism means

Pleochroism is a directional difference in the colours transmitted by a mineral. The colours arise from wavelength-selective absorption and vary with viewing orientation.

How to observe

View a suitable transparent gem with the dichroscope against a bright, neutral light source. Rotate the gem and instrument to compare the colours in the adjacent windows.

Use with other tests

A strong dichroic or trichroic response can support identification, but absence of a visible difference does not prove optical isotropy.

Illustrated guide

Pleochroism: selective absorption — illustrated gemology guide
Pleochroism: selective absorption
Pleochroism in iolite
Pleochroism in iolite
Facet doubling in zircon and moissanite — illustrated gemology guide
Facet doubling in zircon and moissanite
Two polarised views through a dichroscope — illustrated gemology guide
Two polarised views through a dichroscope

Frequently asked questions

Which gemstones may show strong pleochroism?

Examples include iolite (cordierite), tanzanite and some tourmalines, depending on orientation and composition.