Question

Question #b7edc

Answer 1

Because the sand is almost pure silicon dioxide `SiO_2`, or it is only mixed with colourless other compounds.

Explanation:

Sand is mostly `SiO_2`, but even small admixtures of other compounds may give a strong colour, e.g. iron silicates may give a red-brown colour.
Other silicates, like aluminium silicates, are also white.