The Ivory Road is a reflection of the real life Silk Road, so probably precious metals to horses, weapons, woolen goods, glassware, gold and silver, and precious stones and jewels. At least that's what google provides. Well, it seems I've been Mathilde'd.
If you're looking for distances, here's an Old World Map that Boney uses with a legend provided. You can measure the distances all the way up to the Mountains of Mourne where it ends, because Cathay never got a detailed map until now with Total War (and even then it's incomplete). Mountains of Mourne has a map but there is no legend provided: