Final answer:
The index of refraction remains constant for a gemstone regardless of its fashioning and is a key factor in determining its visibility when submerged in water. Diamonds, with their high index of refraction, exhibit total internal reflection, which prevents them from becoming invisible when wet, unlike other gemstones with similar refractive indices to water.
Step-by-step explanation:
A property that remains constant no matter how a gem is fashioned is its index of refraction. This physical property is characteristic of matter that is not associated with any change in its chemical composition. For example, a diamond has a particularly high index of refraction and exhibits total internal reflection, which is why it sparkles and does not become invisible when submerged in water, unlike other gemstones with lower indexes of refraction. This property also means that light has trouble escaping the gem due to the small critical angle with air, resulting in a bright sparkle as light is concentrated before exiting.
Therefore, if a ring with a colorless gemstone becomes invisible when dropped into water, it suggests that its index of refraction is similar to that of water, indicating that the gemstone is likely not a diamond. Conversely, a diamond would still be visibly sparkling due to its total internal reflection.