Meh, limited trade with Earth is certainly in the cards; the question of "how limited" depends on a lot of factors, but particularly their return launch costs. Even simple "Martian rock", sold as collectables or decorative stone, in small quantities could fetch tens of thousands of dollars per kilogram. Collectables markets and luxury goods markets ("Oh, the foyer in your palace is granite from Tuscany? How quaint - my foyer is from Mars") are very real things. But one order of magnitude difference in return prices equates to multiple orders of magnitude difference in the size of the market. Likewise, what exactly is available will also affect the value. A brittle sandstone for example isn't going to get the same market for the same price as big chunks of agate. We don't know what all will be found on Mars, but the presence of hydrothermal systems is encouraging; they're associated with quartz, calcite, chalcedony (agate, onyx, etc), zeolites, opal, etc. The jewelry market would be excellent to be able to break into, in terms of the scale versus what they pay per kilogram.