Where does gravel come from?

My matatu stops at Kamokya (Ka-moh-cha) market everyday next to a several piles of marble stones, pebbles and slabs. I normally sit near to a window, and I idly look at the marble and admire its beauty.
One beautiful Friday afternoon, as usual, we stopped at Kamokya market and I lost myself in the beauty of the marble piles. Large slabs, small slabs, fist-size rocks, penny size rocks and finally small gravelly pebbles.
As a designer, I appreciate the colour, shapes, sizes, repetition of design etc. We normally have to wait at Kamokya for a good while, for other passengers.
This time a noise, a little metallic 'tak,tak,tak', brought me back from my usual daydream to the land of Marble. My eyes followed the direction of the noise, and today I was able to see how all that marble was created.
A man was sitting next to one of the piles and chipping away at the little stones until he had fine pieces of gravel.
I remember about ten years ago, a similar scene in Brazil. There I would lose myself in the beauty of the pavements with the 'Copacabana' black and white wave patterns. Then one day also at a bus stop, in Salvador Bahia, I saw how these patterns were created. Men would go down on their hands and knees and put each little stone in place. I was horrified then. 'That was slave labour!' I had thought, 'How could a man go down on his hands and knees to produce a pavement in 1994! '. Today however, a little more grown-up and maybe hardened by world travelling, I thought almost nothing of this scene, a man turning a slab of marble into gravel, by hand in 2005. It only reminded me of time when I valued human labour so much more.
The matatu had filled up and we pulled off, and my view changed from marble to houses, banana trees etc. I would be able to forget about the man and the gravel till another day.