Zirconium is an automotive company in Grand Theft Auto IV, the only vehicle they produce is the Stratum. The Zirconium Building in Liberty City is modeled after the Chrysler Building but the automotive company Schyster appears to be based on Chrysler. How the Stratum is similar to a Honda Accord Wagon might suggest that it's based on Honda, however there is already a car manufacturer based on Honda (Dinka) and it should be very unlikely that a foreign automaker is headquartered in a classic American landmark. Zirconium could also be based on Mercury since they're both elements, though Mercury wasn't named after the element. The Stratum could be meant to emulate a Mercury Sable station wagon, but strangely enough, one already exists in the game, as the Solair (manufactured by Willard, which also seems to be partially based on Mercury). It can also theorized that the Stratum is a captive import if it is indeed not a domestic vehicle.
Zirconium could also be a metaphor for expensive looking vehicles built with poor quality as the element zirconium can be used to make cheap diamond substitutes. The logo appears to be a crystal (probably a zirconia crystal).