W0 -no visible oxidation of metal or sulfide but a limonitic staining may
be noticeable in transmitted light. Fresh falls are usually of this grade,
although some are already W1.
W1 - minor oxide rims around metal and troilite and minor oxide veins.
W2 - moderate oxidation of metal, about 20-60% being affected.
W3 - heavy oxidation of metal and troilite, 60-95% being replaced.
W4 - complete (>95%) oxidation of metal and troilite, but no alteration of
W5 - beginning alteration of mafic silicates, mainly along cracks.
W6 - massive replacement of silicates by clay minerals and oxides.

Wlotzka, F., "A Weathering Scale for the Ordinary Chondrites". Meteoritics,
vol. 28, no. 3 (July 1993), p. 460.

(Thanks to Piper Hollier for providing me this information)