Final answer:
A green garnet with a hardness of 7 and a refractive index of 1.8 is likely a type of grossular garnet known as Tsavorite. This question pertains mainly to gemology but is answered through principles of optics in physics.
Step-by-step explanation:
The question pertains to the properties of a green garnet, specifically referring to its hardness and refractive index. A green garnet with a hardness of 7 is likely to be a type of grossular garnet referred to as Tsavorite, which is well-known for its bright green color. The refractive index mentioned, 1.8, is consistent with the refractive indices of garnets, which range typically from about 1.72 to 1.94.
Inquiring about the identity of a mineral based on its hardness and refractive index suggests the topic falls within the realm of gemology, a subfield of geology, that often involves these two properties to help identify and characterize gemstones. However, given the nature of the question involving indices of refraction, the answer relates closely to principles of optics in physics, where the refractive index is a measure of how much light slows down and changes direction when entering a material.