Prime island

I made this graphic by looking at the primes smaller than 550,000. What I did was start at the center and with the number 7; I went left, up, right or down if the next prime ended with 7,1,3, or 9 respectively. Every 2200 numbers I would randomly change the color. It stayed closer to the center than I thought it would. To me I'm struck by how much it looks like geography.