For anyone finding atmasphere's discussion of the bearings used in the Triplanar important in their consideration of tone arm design, or even just interesting, let me direct you to the Wikipedia entry (I don't know how to attach a link---I gotta learn how to do that!) on Silicon Nitride. One section in the entry is specifically about it's use in the making of bearings. NASA chose Silicon Nitride bearings for the main engines of the Space Shuttle for several reasons, one of them being that they are much harder than bearings made of ANY metal, no matter the hardness rating.
Geoffrey Owen of Helius Designs will be using them in his new TOTL tone arm being readied for introduction next year, but is also offering them as an upgrade on the current Omega arm. The standard Omega has Tungsten bearings, the Silver Ruby Omega, duh, Ruby ones, which as Ralph pointed out are easily damaged. Geoffrey used them because they are so much smoother than any metal (look at even the hardest grade of any metal bearing under a high-power microscope---it has quite a rough surface, no matter the grade; part of Geoffrey's business is in laser optics for use in space exploration telescopes), with far lower friction and, as Geoffrey told me, noise, the level of which is a major design criteria of his in making tone arms.