Physics
The basic reason for the extraordinary sparkle of suitably cut diamond is that

It is very hard
It has well defined cleavage planes
It has a very high transparency
It has a very high refractive index

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Physics
Why does the sea appear blue in colour?

Interference of the sun light from water surface
Scattering of the sunlight by water molecules
Refraction of the sun light
Reflection of the sun light from water surface

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