The term "Diamond Omega" is a specific classification within the fictional Omegaverse presented in the BL manhwa (boys' love comic) titled Diamond Dust.
In this story, a Diamond Omega possesses two key, rare characteristics that set them apart from regular Omegas in that universe:
Scentless (initially): The protagonist, Seo Yeehyeon, lives his life believing he is a Beta because he does not exhibit the typical pheromones of an Omega.
Unique Pheromones: His true Omega scent is a "sweet, addictive pheromone" that can only be perceived by his destined "Golden Alpha" partner, Liu Weikun, and no one else. This exclusivity is the basis of the story's main plot, as the unique scent acts as a catalyst for their relationship and the Alpha's obsession.

Is he a diamond omega? Maybe his results are wrong and it ks hard to tell due to rarity. It's the same with Kiss Me series for dominant omega.