This was my first Cob project, which I learned from books, web, talking with others and from just doing it!
The clay is from the surrounding area and is generally pure enough without needing to be filtered.
The sand is from a local river bed. Normally I would use sharp sand, but there was enough 'bite' to make it usable.
The fibre is Vettiver. This was added to the mix as short-chopped pieces (5 to 10 cm) and also as full-length fibres (up to 1 metre long) added during the building process.
'Dry' mix proportions for the cob:
Sand 80%
Clay 10%
Fibre 10%
These proportions varied with the different layers, but always mainly sand.