Why do some grains appear black?

Grains in thin sections appear black for one of three possible reasons.

Opaque mineral

The grain is opaque and doesn't allow light to pass through.  In a thin section, this property is fairly well restricted to minerals that would have a metallic appearance (metallic luster) in hand specimen.  This includes native metals such as copper, gold, or silver, metallic ores that are typically sulfides or oxides, or graphite.

You can tell if a black-appearing mineral is opaque by making sure you are looking through only one (or no) polarizing filter.
 

The other (following) effects occur only when both polarizing filters are used.

Extinction

This has nothing to do with life-forms from the past.  Extinction refers to the effect in which mineral grains go black once every ninety degrees as the microscope stage is rotated.  If you have access to a polarizing light microscope (PLM) or are able to access the Virtual Microscope web site (fast internet access desirable), you can observe this effect.  Many of the grains in the gallery appear black for this reason.  If the photomicrographs had been taken when the stage was at a different angle, a different set of grains would have appeared black.
 

Isotropic material

Some materials do not display interference colors when both polarizing filters are used, because they have only one index of refraction (see interference colors discussion).   These materials include any liquid, as well as both natural and artificial glass (which are actually "supercooled liquids").  These materials also include a few minerals such as garnet and diamond.  Instead of showing interference colors, these substances appear black and will remain so throughout rotation.
 

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