When viewed under plane polarised light, some mineral grains are more clearly defined than others. They have heavier, darker edges. These minerals have higher relief.
Relief is determined by the difference in the refractive indices of the grain and mounting medium. The mounting medium typically has a refractive index (R.I., ni) of ~1.54. A mineral with a low refractive index, such as quartz (R.I., ni ~1.5) has low relief because it has a refractive index very similar to that of the mounting medium. A mineral with a high refractive index, such as garnet (R.I., ni ~1.7) has high relief because it has a refractive index much higher than that of the mounting medium.
Minerals displaying the lustre of resin or of smooth-surfaced plastic are described as resinous. Amber, for example, has resinous lustre.
Three intersecting cleavage planes. A mineral with rhombohedral cleavage will break up along its cleavage planes into perfect rhombohedra. Calcite, for example, has rhombohedral cleavage.