In Calcite nomega > nepsilon, 1.658 versus 1.485. In other minerals, e.g. quartz, nomega < nepsilon , 1.544 versus 1.553.
This difference in this refractive index relationship provides the basis for defining the optic sign of uniaxial minerals.
nepsilon refers to the maximum or minimum index of refraction for the extraordinary ray, the value recorded in the mineral descriptions in the text.
nepsilon' refers to an index of refraction for the extraordinary ray which is between nomega and nepsilon.
For uniaxial minerals any orientation will provide nomega, but only one orientation, cut parallel to the c-axis will yield nepsilon maximum. This orientation is the one which exhibits the highest interference colour as delta (birefringence), is greatest, and therefore retardation (DELTA) is greatest.