Onyx (nicknamed Jet), from Jackson's Whole, was the youngest of Baronne Cordonah's famous living Jewels, and the only male. In coloration, he was jet black and silver. While the other Jewels were gengineered primarily from the Baronne's genome, Jet's Y chromosome was taken from the Baron.
When House Prestene overthrew House Cordonah, Onyx fled to Escobar to stay with his older even-brother Amiri among the Duronas. When the rest of the family went to Barrayar to collect Rish and Tej, they stopped by way of Escobar and collected Amiri and Onyx as well.