Heritage offers a very informative written description on the mystery surrounding the coinage of Nigrinian (see this link GO):
Rare coins bearing a youthful male portrait with the inscription DIVO NIGRINIANO have long puzzled numismatists and historians, who could find no mention of any prince of this name in the historical sources. The discovery in the early 1900s of an inscription naming Nigrinian as a grandson of the Emperor Carus (AD 282-283) narrowed down the possibilities to Carus’ two sons, Carinus or Numerian, both of whom ruled jointly with their father and by themselves after his death. Given that Nigrinian’s posthumous coinage is confined to the Rome mint, it seems the most likely candidate for his father is Carinus, who presided over the West from Rome, while Numerian’s brief reign was spent in the East. Nigrinian’s mother might then be Magnia Urbica, who was proclaimed Augusta in AD 283 and honored with coinage of her own. According to this scenario, Nigrinian was likely born in 283, but by the latter part of AD 284 he had died and was deified by the senate, no doubt on the order of Carinus. However, coins struck to mark his deification depict Nigrinian as a boy of at least seven or eight. Either this is artistic license, or Nigrinian was born much earlier. The Historia Augusta claims Carinus “took nine wives in all, and he put away some while they were still pregnant.” Nigrinian thus might have been a product of one of these earlier marriages. Until more definitive evidence is uncovered, the mystery of Nigrinian’s true parentage must remain unsolved.
Whoever his mother was, Nigrinian’s coins are of considerable rarity, indicating that striking commenced shortly before the downfall of Carinus’ regime early in AD 285 brought it to an abrupt halt. His successor, Diocletian, condemned the memory of Carinus and no doubt revoked the deification of Nigrinian, who thus became a long-lost footnote to Roman history. This remarkable billon aurelianianus bears a stunning half-length bust of Nigrinian that can only be described as a masterpiece of numismatic portraiture from this troubled era. The youth is shown “heroically nude” and with one shoulder raised, perhaps indicating it was modeled on a statue of Nigrinian in an oratorical pose.