In the third edition of “Fedorov’s Universal Method,” in Chapter VII — “dispersion research” — V.V. Nikitin describes idocrase, which has a sharp birefringence dispersion. In addition, it also has the property that within the same grain, in different parts of it, the magnitude of birefringence is different. Due to this, the interference coloring is not uniform throughout the idocrase grains. In sections close to parallelism to the optical axis — the quadruple axis of symmetry of the crystal — the colors are arranged in stripes parallel to each other: the central part of the grain has a white interference color, followed by a stripe with a mastic yellow color, then with a purple-red color, and finally, the edges of the grain are colored in a purple-violet color.
