Hardness is water is made up of calcium and magnesium (natural minerals). The hardness cannot be seen with the naked eye, but can be detected through testing procedures.
What Does Hard Water Look Like?
I didn't know what hard water was, so I looked it up and here is an image I found. The image is of hard water that caused calcification on this faucet. 
Well really that was a kinda stupid question but any way ;D hard water normally looks like ice or ice poles