Papakōlea Beach is located in a bay half circled by Puʻu Mahana, a tuff ring formed over 49,000 years ago and associated with the southwest rift of Mauna Loa. Unlike cinder cones, tuff rings consist mostly of volcanic ash produced by violent interactions of magma with groundwater (Diamond Head, on the Island of Oahu, is another example of a tuff ring). Since its last eruption, the tuff ring has partially collapsed and been partially eroded by the ocean. The beach is sometimes named after the tuff ring, and sometimes after the area of land called Papakōlea, which comes from papa kōlea, which means plover flats in the Hawaiian language. Papakōlea is the area near the crater where Pacific golden plovers (Pluvialis fulva) are sometimes seen in winter.
The fragmented volcanic material (pyroclastics) of the tuff ring contains olivine, a silicate mineral containing iron and magnesium, also known as peridot when of gem quality. Olivine is a common mineral component of lava and is one of the first crystals to form as magma cools. Olivine is locally known as “Hawaiian Diamond” and is notably found in Oʻahu’s famous Diamond Head landmark. The source of the green coloration of the beach sands is due to the olivine crystals (whose green color is due to ferrous iron) which are winnowed from the eroding headland by the action of the sea.
Olivine, being denser than the enclosing ash matrix, tends to accumulate on the beach whereas the less dense volcanic sand is swept out to sea. Elsewhere on the Big Island, olivine is enclosed in lava rock, rather than volcanic ash, so the olivine is not easily freed from the enclosing rock and tends to weather away rather than accumulate and concentrate as beach sand.
Olivine Crystals in Lava Rock. This is the source of the green sand found on the green sand beach
Although these crystals are eventually broken down by weathering and chemical action and washed away, the constant erosion of the tuff ring ensures a steady supply of sand for the foreseeable future. Eventually, however, the supply will run out and the beach will look like any other.
According to en.wikipedia; theactivetimes.com. Source of photos: internet