for the 4 months before setting up this tank, I had several large petri dishes under bright light, that I seeded with a few different types of algae. I had 2 green algaes going, and a brown algae one, as well as 2 vases of lite greenwater growing in the window. I added doses of these cultures daily for the first week... and it really sped up the algae growth on the rocks.
You can certainly start rocks in sunlight... a bin with water and fertilizer in the sun will do the trick.