Olivine is most well known for its distinct green color and vitreous to glassy luster. The luster of a mineral refers to the way light reflects off its surface. Luster falls on a spectrum from metallic to dull. Metallic minerals like gold and hematite reflect light like a metal, while non-metallic minerals have a vitreous, pearly, silky, or dull luster. The green color of olivine combined with its vitreous to glassy luster make it easy to identify.
Olivine exhibits a vitreous luster when it has a smooth, glass-like surface. Vitreous comes from the same Latin root as "vitrum" meaning glass. When light hits a vitreous mineral surface it reflects and refracts cleanly as it would off a pane of glass. This makes the mineral appear shiny and lustrous. Freshly broken or cut surfaces of olivine exhibit a perfectly vitreous luster.
As olivine weathers and oxidizes over time, its luster changes from vitreous to greasy or dull. This is because chemical weathering processes alter the crystal surface. Tiny pits and scratches diffuse the reflection of light rather than allowing it to reflect cleanly off the surface. A greasy luster appears somewhat oily, while a dull luster means the mineral reflects very little light. Beach sand often contains weathered olivine grains with these altered lusters.
Another factor that affects olivine's luster is the abundance of microfractures. Olivine has a perfect basal cleavage parallel to the orthorhombic crystal structure. This means it tends to break along flat planes at right angles that reflect light. But olivine also often forms with tiny fractures not oriented to the cleavage plane. The more fractured the crystal, the more the luster shifts from vitreous to greasy or dull. Microfractures scatter light in more directions.
Finally, the grain size of olivine impacts luster. Coarse-grained olivine shows a stronger vitreous luster because it has larger crystal faces for light to reflect from. Fine-grained olivine rocks like peridotite appear duller. The smaller mineral grains don't shine as brightly. Larger olivine phenocrysts in basalt show a stronger glassy luster.
The green color and vitreous to glassy luster make olivine easy to identify in hand samples. With a little practice, one can readily distinguish it from other common minerals like quartz, calcite, pyroxene, and amphibole that share rock types with olivine. The distinctive reflective sheen off a smooth olivine surface cannot be mistaken.
