A sample of element 247
The 247th element was a naturally-occurring, stable, transuranic element discovered in 2371 by the crew of the USS Voyager. It was initially detected in the rings of a class D planet in the Delta Quadrant. It was so described because it was the 247th element discovered by the Federation. Its name remained unclear even as late as Voyager's return to the Alpha Quadrant.
Properties
Harry Kim noted that it had an unusually high atomic mass, being possessed of "over 550 nucleons". Citing its natural occurrence and stability, Kathryn Janeway emphatically called it "a first". B'Elanna Torres and her captain theorized that, if it could be mined, it could be used to fit probes with casings that would allow them to travel into the heart of a sun, or to create ultra-thin reactor casings.
The element was formed as a by-product of the decomposition of Vhnori bodies. Citing deep respect for the Vhnori deceased, Chakotay influenced Janeway to avoid using even simple tricorder scans to gather detailed on-site data about it. Consequently, the element was never mined, used, or even studied in great detail by the Federation. Knowledge about its atom was limited to scans done by Voyager at an unknown distance, and one cursory tricorder scan onsite. Since a full analysis was never completed, even basic details – like the atomic number and precise atomic mass – were unknown. (VOY: "Emanations")