In trying to achieve a long lasting, low maintenance drivetrain, I've read statements about aluminium oxide being the source of the black colour of chain dirt and also being highly abrasive:
Aluminum oxide from chainrings makes chain dirt black - and it is very hard and abrasive. Grinding wheels are made of aluminum oxide!
— John Allen on sheldonbrown.com
Others state that pretty much all the abrasives (including the aluminium oxide) come from the road:
I believe almost all the grit comes from the road. Aluminum is very common in most soils, in the form of aluminum oxide, which is a pretty effective abrasive.
— Frank Krygowski in rec.bicycles.tech
My question is this:
How much of the dirt/abrasive on a chain comes from the chainrings, and how much does it really affect chain wear?
My feeling is that the aluminium oxide particles ground off the chainrings would be of very low diameter, and thus wouldn't have a significant abrasive effect compared to the larger particles of sand and dirt thrown up from the road/path itself. Obviously this brings up questions as to the lubricant used (as a sticky lube will attract dirt) and riding conditions, but for the sake of this discussion, I'd like to consider a bicycle which is regularly maintained and ridden in all weather, primarily on made roads.
To bring this into the realm of the practical, my reason for asking is that if the cog material truly does affect chain longevity, I would consider changing to steel cogs (not currently considering the other advantages/disadvantages of steel, which have been covered elsewhere).