Oneuli means "dark sands" which aptly describes the beach's salt and pepper mixture of black and white sand. After years of abuse from the ocean waves, the nearby Pu'u Ola'i Cinder Cone's deposits were ground into black sand and washed up on the beach.

Combine that with crushed seashells and you have a lovely mixture. One of the highlights of this beach is the snorkeling. 

A large lava shelf off shore makes swimming conditions not ideal, but the coral-filled bay makes up for it. This is a favorite location for honu (green sea turtles) and occasionally manta rays and small sharks. The best locations for snorkeling and scuba diving are at the north end of the beach following the length of the beach.

During calm seas you may be able to snorkel all the way around the cinder cone, but be mindful of currents and come up to check if you are still on track. This is also a good place to launch a kayak. 

According to hawaii-guide.com