What is Forsterite?
Forsterite is the magnesium-rich olivine end-member, important in mantle rocks and prized when transparent pale green material is suitable for gem cutting.
Mineralogy and structure
Reference formula: Mg2SiO4. Forsterite has the orthorhombic olivine structure and the ideal formula Mg2SiO4. Iron substitutes for magnesium toward fayalite, so natural olivine commonly lies between the two end-members.
Colour and appearance
Typical colours include colourless, white, pale green, yellow-green. The usual prismatic, granular, massive habit and vitreous lustre are useful descriptive features, but colour alone is not diagnostic.
Formation and localities
Forsterite forms in or is associated with peridotites, magnesium-rich marbles, mafic and ultramafic rocks, and some meteorites. Important documented localities include Pakistan, Myanmar, Italy, Norway, the United States, South Africa and meteorite finds.
Collecting and care
Usually durable for display and jewellery, although fractures and inclusions can weaken transparent crystals; avoid confusing a colour name with a confirmed composition.
Forsterite as a mantle mineral
Forsterite is especially useful in a guide because it connects a hand specimen with the Earth's mantle and with the olivine family seen in meteorites. Collectors may use its pale green colour as a symbol of renewal or natural vitality, but these are personal or cultural meanings.
Traditional associations
Olivine-group gems are sometimes linked with August birthstone traditions, while other lists assign them to Leo or Virgo. These associations vary and do not identify Forsterite.
