Very funny k97734kg.
I suspect it's just one of those things alex. I'm not familiar with the Intercon Beach but I doubt if it was "designed".
Some of the stones reflect all wavelengths of visible light and therefore appear white to the eye. Others absorb all visible wavelengths and appear black. The difference has to do with the properties of the molecules of the individual elements that comprise the stones. In other words, while they may all look like "the same type of stone", they are not.