There seems to be an urban legend that the only beach in the world having green sand is near the southern tip of Big Island (Hawaii). An alternative version is that it is one out of two. The other being in Guam (Talofofo Beach).
I have to first make it clear that this article is about the green beach sands that are composed of mineral olivine. Lots of other minerals can form green sand beaches also but we exclude them for now.
I do not know for sure about Guam. I have a very nice sand from the Talofofo Beach but it is composed almost exclusively of magnetite. But there is no doubt that olivine sand could be there. Guam is volcanic island located on top of the Mariana Island Arc right next to the famous Mariana Trench that reaches almost 11 kilometers in depth and is the deepest part of the world’s oceans.
The famous green sand beach in Hawaii is called Papakolea or Pu’u Mahana Beach. As much as I know it really has few competitors in terms of purity and freshness of the olivine sand. However, it is definitely not correct to say that it is the only one. The absence of evidence is not the evidence of absence.
There is no need to go very far from the Big Island. Diamond Head is a tuff cone in Oahu that constantly feeds the sand beach right next to it with fresh olivine. The sand there is not as bright but it is definitely composed mostly of olivine and it is green.
It is interesting to take a look at the Pu’u Mahana and Diamond Head volcanoes. There are some striking similarities. They are both located right next to the beach. They both contain lots of olivine. They are both composed of easily erodable pyroclastic sediments. They are both high and steep which speeds up the erosion.
Why is this needed for the green sand beach to form? Because olivine is extremely unstable mineral in the atmospheric conditions. The transport route needs to be short and erosion fast to ensure that most of the olivine makes it to the beach and there is a constant supply of fresh material.
All right, we have Diamond Head Beach in addition to Pu’u Mahana but it is still only two and both of them in Hawaii. It may come as a surprise to many but olivine is not rare in sand. I would even say that it is an essential component of sand in many oceanic islands or volcanically active regions. Canary Islands, Iceland, Galapagos Islands, and Cape Verde are just a few names where olivine is a common constituent of many beach sands.
I especially love a sand sample from St. Lawrence Island in the Bering Sea. The climate there is not quite comparable to the Caribbean. That’s perhaps the most important reason why this island is not very popular among tourists. The local Yupik people are quite protective about their island as well but I am lucky to know two of them. They sent me a very nice sample from the northern coast of Sivuqaq (St. Lawrence Island in Central Siberian Yupik). This sand sample is not as green as are the samples from Hawaii because it contains lots of lithic material but olivine definitely dominates among the single minerals and gives greenish hue to the sand.