A comprehensive experience for such a short detour when driving to or from Volcano, as part of a trip south of Kona or as a trip in its own right.
Drive down South Point Road to a parking lot, preceding a ~3 mi rough dirt road / trail to the green sand beach. This is handled one of 3 ways:
1) Rent a Jeep 4x4 - a generally useful vehicle in a place like Hawaii, it is also essential to easily access several gems on the island. The road here is certainly heavily-rutted and muddy, however, the right suspension and ground clearance make it a very doable drive.
2) Ride with locals - there is typically a group of locals seeming to have a barbecue near the junction between the paved parking lot and the dirt trail. They have beat-up trucks that they use to shuttle people to and from the beach. Expect to sit in the bed of a pickup on cushions or a mat. My experience found them to be very friendly and efficient, albeit the ride was a (enjoyable) roller-coaster experience. They will ask for $15 R/T pp (generally as part of a group of people, i.e., my group of 4 was quoted $60 - this can be reduced to $10 or less pp quite easily. We ended up paying $30 altogether, which is more than fair for the service. Great interaction with locals who speak the local creole amongst each other and really do seem to care about your experience.
Driving would take 15-20 minutes each way, perhaps a bit quicker when riding on one of the trucks.
3) Hike - self-explanatory, a pretty reasonable trail that will take about 1 hr. Direct access to the beach requires climbing down a staircase-like trail that can be a bit slippery (common to all methods, vehicles cannot do this last part).
The green sand beach is in a very scenic cove, with fascinating textures on the rock walls surrounding the beach. As far as the green sand itself, it is certainly unique and also seems to have a more mud-like consistency than typical sand. The surf is fairly rough, often quickly changing strength (some waves reach close to the back wall of the cove, although this area is generally beyond the reach of the water). Swimming far from shore would not be recommended, but a dip at the edge of the water is no issue. Semi-skilled surfers could attempt the waves, as I saw several doing.
Not a negative in my opinion, but there are no beach facilities available, i.e., showers, changing rooms, restrooms, etc. If you ride with the locals, they will likely let you use a makeshift outhouse near the picnic tables where they hang out.
Overall a must-do, particularly if passing nearby.
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