You need to be more specific on your location and era. The ballast will vary.
Shortly after the beginning of the twentieth century, the SP converted all locomotives operating in California, Nevada, and Oregon to fuel oil, so there would be little source of cinders coming from burnt coal.
Main track ballast on the SP was normally crushed rock.
Ballast for the SP lines in the upper-northern portion of California and southern portion of Oregon came from volcanic cinders. The SP had a quarry for this purpose at Bigmarsh, renamed Chinchalo. (I believe the Arizona Rock & Mineral Company sells a black-colored ballast for SP modelers.) However, at some point of time, crushed rock ballast was used in this area. Slag has also been used on some portions of the SP.
I wouldn’t concern myself with sub-ballast which would consist of materials directly at hand at the location. Ballast will cover it.