SSTs are sometimes called "marble-top tubes" or "red top tubes", referring to the stoppers which are either gold or red-gray. Trademarked versions include Tyco Healthcare "Corvac" tubes.
They contain a special gel that separates blood cells from serum, as well as particles to cause blood to clot quickly. The blood sample may then be centrifuged, allowing the clear serum to be removed for testing.
These tubes cannot be used for therapeutic drug levels because the drug will diffuse from the serum into the gel, causing a reduction in measured drug level. The gel in SST II tubes (which appears slightly less opaque) is supposed to have less effect on drug levels in serum.