The first three rank much higher than the others. If you can manage to get to those 3 there is a fantastic selection of rock and gravel.
Morgan's Masonry, San Ramon and Antioch- Plenty of gravel, lots of rock the right size to call boulders in the aquarium. LOTS of nice stuff.
LH Voss, Dublin- Good selection of unusual rocks.
Mount Diablo Landscape, Concord- Best selection of unusual rocks, often carries thin black slate for backgrounds and a lot more.
Diamond K, Lafayette- Better for large landscape boulders, but if you are anywhere near, go see it. Lower on my list, though for aquariums.
Brickyard, Concord. Has graded sand in bags. Lapis Lustre brand, a sort of golden and silvery sand in different sizes. Occasionally has interesting rocks.
Near Brickyard are a couple of swimming pool suppliers for pool filter sand.
There are a couple of rock yards in Brentwood, if you are going out that far, but mostly they are more concrete suppliers, not decorative rock.
MacAlvey's Nursery, Martinez. Small selection, but if you are driving by it, worth a stop.