In contrast to cis-1,4-polyisoprene (IR) that is manufactured in chemical manufacturing facilities, natural rubber (TSR 20) is produced by Hevea brasiliensis trees on rubber plantations. Natural rubber is a major ingredient of the skim rubber that is calendared onto the brass-coated steel cords of the steel belts of vehicle tires. As a direct consequence of its source, natural rubber contains low levels of impurities. Therefore, one would expect to find these natural impurities in the steel belts of tires. The impurities actually found in various lots of natural rubber have been characterized. And, used tires have been dissected and analyzed to determine if any of the types of impurities can be linked to specific fracture surface features. The results of the study will be presented.