The main gem for Mars is a red coral. It should be at least three carats in size, set in silver, or a combination of gold and copper, and worn on the index or ring finger of the right hand. As it is a common and not too expensive gemstone, no substitutes are required, though carnelian can be used in this respect (generally as a large pendant). Pink coral can also be used, and has a more balancing or subduing, but less strengthening affect on Mars energy than the red.
Such gems should be put on first on a Tuesday, when the Mars is waxing, preferably when Mars is in its own sign or exalted. Friendly signs, particularly those of Jupiter, are also good.