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This new comprehensive in situ mineral-chemical characterisation of the Happy Jack uraninite has discovered additional information regarding matrix effects and trace element homogeneity, relevant to proposals that it could serve as a reference material (RM). On the LA-ICP-MS instrumentation used, there was an absence of discernible matrix effects relative to the silicate glass NIST SRM 610. Lanthanides and Y are found to be very homogeneously distributed in Happy Jack uraninite, Zr, Nb and Ti mass fractions reproducible but only within individual fragments, and other elements still generally heterogeneous. This means that the Happy Jack uraninite can serve as a secondary RM for quality control in studies of natural uraninites. In terms of absolute accuracy and intermediate measurement precision, the Happy Jack uraninite can be used for the homogeneous elements. For elements that are homogeneous within individual fragments only, intermediate measurement precision can still be evaluated, while information values will be obtained for the generally heterogeneous elements. Two distinct groups (high vs. low Zr) were distinguished to exist among different Happy Jack fragments in association with minor variation of REE mass fractions, which possibly explains the observed (heavy) REE discrepancy between in situ laser ablation and bulk solution ICP-MS analyses., (© 2022 The Authors. Geostandards and Geoanalytical Research published by John Wiley & Sons Ltd on behalf of the International Association of Geoanalysts.)