What is a defining feature of said glicosope?

The defining feature of a glicosope is its ability to allow for selective color absorption. This means that it can filter light in such a way that only certain wavelengths, or colors, reach the viewer while absorbing others. This property is particularly useful in gemology and mineralogy, as it helps identify gemstones and their characteristics based on how they absorb and transmit light. By selecting specific wavelengths, a glicosope can provide insight into the composition and quality of a colored stone, making it an invaluable tool for gemologists.

The options regarding reflecting all incoming light or only transmitting blue light don't accurately describe the functionality of a glicosope, as it is not designed to reflect or transmit light in such a limited manner. Additionally, being opaque and unilluminated would not be a defining feature, as this would contradict the purpose of the instrument, which is to analyze and observe light interactions with gemstones.