1 mm diameter, stainless steel. Some version 'Millennium' edition portafilters were not manufactured with the groove to accept this spring, so don't be surprised if you unit doesn't have a place for it. However, the 'pre-Millennium' did, very rarely come with the groove for a spring.
WARNING: This product can expose you to chemicals including lead, which are known to the State of California to cause cancer, birth defects, or other reproductive harm. For more information, go to www.p65warnings.ca.gov.