The extraordinary sparkle of a suitably cut diamond is primarily due to its very high refractive index, which is why option 2 is correct. The refractive index is a measure of how much a ray of light changes direction when it travels within an object. For a diamond, this is high, meaning light bends considerably, undergoing total internal reflection causing the sparkle or brilliance we observe.

Option 1, high transparency, is not the primary reason, although it plays a part as it allows light to enter and pass through the diamond. However, without a high refractive index, the light wouldn`t interact significantly within the stone to create the sparkle.

Option 3, hardness of diamond, contributes to its durability and ability to retain its cut and facets, but doesn`t directly impact its sparkle.

Option 4, well-defined cleavage planes, relate to how a diamond can be split along certain directions. It has little to do with making a diamond sparkle. While it can affect how a diamond is cut, it’s the resulting interactions between light and the internal facets, controlled by the refractive index, that create the sparkles.