Ruthenium

Ruthenium has the symbol Ru and atomic number 44. It is a rare transition metal in the Platinum Group of metals.

It is extremely valuable ($831,100 / kg) and can be shipped back to Earth at a profit.

It is a very valuable catalyst and would be useful in the petrochemical industry on Mars which would build up more complex petrochemicals from Carbon dioxide and water feed stocks. (Alternately from plants and water feed stocks.) It is also used in electronics as it is resistant to oxidization, and it hardens other platinum group metals.

Ruthenium has been locked away deep in the earth, but is relatively common in metallic asteroids. If Mars is supporting an asteroid mining industry, Ruthenium may be concentrated and refined on Mars or in Mars orbit.