Final answer:
A blue garnet with a hardness of 7 and a refractive index of 1.8 is most likely a type of gemstone or mineral. The hardness and refractive index are indicators of its properties.
Step-by-step explanation:
A blue garnet with a hardness of 7 and refractive index of 1.8 must be classified based on its properties. In this case, the hardness of 7 indicates that the garnet is relatively hard, which is often associated with minerals like quartz and topaz.
The refractive index of 1.8 suggests that the garnet has a moderate ability to bend light. Based on these properties, the blue garnet is most likely a type of gemstone or mineral.
It's worth noting that there are natural and synthetic variations of blue garnets, and their exact properties may vary. However, the hardness and refractive index can provide a general indication of the type of material.