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The isotopic composition and molar mass M of silicon in a new crystal (code: Si28-33Pr11) measured by isotope ratio mass spectrometry using a high-resolution multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS) is presented using the virtual-element isotope dilution mass spectrometry (VE-IDMS) method. For this new crystal, M = 27.976 950 48 (16) g/mol was determined with urel(M) = 5.7 &times, 10&minus, 9. The &ldquo, X-ray-crystal-density (XRCD) method&rdquo, one of the primary methods for realizing and disseminating the SI units kilogram and mole in the recently revised SI, is based on &ldquo, counting&rdquo, silicon atoms in silicon single crystal spheres. One of the key quantities is the isotopic composition&mdash, expressed by the molar mass M&mdash, of the three stable isotopes 28Si, 29Si, and 30Si in the material highly enriched in 28Si. M was determined with lowest possible uncertainty using latest improvements of the experimental techniques. All uncertainties were estimated according to the &ldquo, Guide to the expression of uncertainty in measurement, GUM&rdquo, The results of the new crystal are discussed and compared with the four previously available crystals, establishing a worldwide limited pool of primary reference spheres of highest metrological quality.