Observations of rocks on the plains show they contain the minerals pyroxene, olivine, plagioclase, and magnetite.  These rocks can be classified in different ways.  The amounts and types of minerals make the rocks primitive basalts—also called picritic basalts.  The rocks are similar to ancient terrestrial rocks called basaltic komatiites.  Rocks of the plains also resemble the basaltic shergottites, meteorites which came from Mars.  One classification system compares the amount of alkali elements to the amount of silica on a graph; in this system, Gusev plains rocks lie near the junction of basalt, picrobasalt, and tephrite.  The Irvine-Barager classification calls them basalts.
Plain's rocks have been very slightly altered, probably by thin films of water because they are softer and contain veins of light colored material that may be bromine compounds, as well as coatings or rinds.  It is thought that small amounts of water may have gotten into cracks inducing mineralization processes).
Coatings on the rocks may have occurred when rocks were buried and interacted with thin films of water and dust.
One sign that they were altered was that it was easier to grind these rocks compared to the same types of rocks found on Earth.
The first rock that Spirit studied was Adirondack.  It turned out to be typical of the other rocks on the plains.