> These meteorites are interpreted as Martian because they have elemental and isotopic compositions that are similar to rocks and atmospheric gases on Mars, which have been measured by orbiting spacecraft, surface landers and rovers.

Basically there was a rare set of clustered properties which were very similar to properties of the rocks and atmosphere on mars as tested by our various landers.

https://en.wikipedia.org/wiki/Martian_meteorite

> The examination found that it is an “olivine-microgabbroic shergottite,” a type of Martian rock formed from the slow cooling of Martian magma. It has a course-grained texture and contains the minerals pyroxene and olivine, Sotheby’s says.

And that magma is probably identified by isotopic composition. Similar to how we know that the Moon used to be part of Earth.

The smell of people's hands touching it repeatedly?

So are you…

To specifically answer the optimal quantity question, the above answer implies that you would keep bringing rocks home until the cost to bring them back is higher than the price you set. To be clear, you could still go out and get more rocks and sell them for less and still make a profit, but you wouldn’t maximize your profit with that strategy. Competitive markets nudge behavior towards providing more product for less profit/cost which is why we love competition.

[0] Planet money did a great podcast on how to actually do this. Here’s a link to a transcript (though listening is probably best): https://podscripts.co/podcasts/planet-money/how-much-for-tha...