dichroscope

A dichroscope is a small optical instrument used in mineralogy and gemology to observe pleochroism, the phenomenon where a crystal or gemstone shows different colors when viewed from different directions. By splitting incoming light into two images, the dichroscope allows a viewer to compare colors along two crystallographic directions of the specimen.

How it works: Most dichroscopes use a Wollaston prism or paired calcite prisms to split light into two polarized beams that travel along perpendicular optic directions. When a gemstone is placed in the viewing aperture, light that passes through the stone is separated into two images. If the stone is pleochroic, the two images reveal different colors; if not, the two images appear the same. The instrument may include an adjustable aperture or analyzer to optimize contrast between the two channels.

Types and design: Modern handheld dichroscopes commonly employ a Wollaston-prism arrangement to produce side-by-side dichroic images. Older or simpler designs may rely on calcite prisms to achieve the same effect. Some units incorporate features to aid alignment and to enhance color separation for easier comparison.

Uses and limitations: Dichroscopes are used to aid gemstone identification and to document pleochroism in stones such as tourmaline, cordierite, iolite, and other dichroic minerals. They help in determining color orientation and can assist in distinguishing between certain natural and synthetic stones. Limitations include weak or absent pleochroism in some minerals, the need for proper stone orientation, and lighting conditions that affect color perception.

History: The use of devices to observe double refraction and pleochroism developed in the late 19th to early 20th century as part of advances in mineralogical and gemological instrumentation.