High relief kyanite surrounded by other minerals of lower relief, some colored (biotite), others colorless.

Relief is what makes the surface of a mineral grain in a thin section stand out even when the mineral is colorless.  It is the result of the contrast between a mineral's index of refraction and that of the mounting medium, usually the epoxy by which the rock chip was glued to the glass slide.

A colorless, low relief mineral will be practically invisible, unless you reduce the light being transmitted by adjusting the iris diaphragm.  Of course, when both polarizing filters are used ("crossed nicols") all minerals in a thin section become visible by their interference colors.

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