It depends on the OP's usage.

Sand is a noncount noun like sugar or milk meaning the plural form and singular form are the same. So the plural of 'sand' is still 'sand' and not 'sands'. Count versus noncount nouns are what determine the usage of much or many.

So there's a problem with the statement you're addressing which probably tipped off your intuition to something not sounding right.

Let's dissect the statement. 'Sand' is a noncount noun as we've already established. This means the plural and singular forms are the same. 'Is' is only used for singular nouns. For example "The car is fast" versus "The cars are fast." This means that sand is being used as a singular noun. The problem arises with the next word "rocks". Rocks is a count noun and is clearly being used as plural here. This is where the conflict is occurs. It should either be "sand is rock" or "sand are rocks". Either of those statements is consistent in plurality or singularity while the original is an incorrect mixture of both.

Or I could be completely wrong about all of this. I took a linguistics course a long time ago and I'm a biology major. ¯\(ツ)/¯