Stainless steel ones, actually. Friction is the big enemy of multiple block systems so ball bearings are used to keep things moving freely – friction loss is kept below 3% as opposed to more than 10% per block with plain bearings. That’s really important for mainsheets and vangs.
Some single block applications don’t require movement under load. For instance, halyard blocks are used to hoist a sail but once that job’s done they just sit around waiting for the next hoist. In that application there is a danger that the balls, plastic ones in particular, will deform under the constant single point load so the large load bearing area of a sleeve might work better.
Stainless steel balls give you low friction and low distortion.
Over on the website there’s an article about using blocks to tame your mainsail, and you can also browse our range of Viadana reinforced stainless steel ball bearing blocks.