This one is also about the Goddess of Pele. Since she is the Goddess of Volcanoes and rumored to be the creator of Hawaii islands, all the lava rocks and black sand is said to be a part of her. Ive heard that it is bad luck to take anything volcano related off of the island. A lot of tourists who didnt know would take home pieces of lava rock or black sand as souvenirs, but I heard many of them ended up returning whatever they took because bad things would happen to them. Some even wrote apologies to the Goddess asking for forgiveness and theres some museum displaying things people sent back, theres even an address to mail back the rocks.
The informant heard this superstition from many of her friends growing up on the island of Oahu. Stories of the Goddess are popular among local folk culture and this particular superstition is one which many tourists should be aware of. It has also become a tourist tale these days since Hawaiian tour guides will often warn those who step onto Hawaii, take only those that are given or come with nothing, leave with nothing.
As with all superstitions, I think those that visit the island, who otherwise wouldve openly taken rocks as souvenirs, would probably second guess themselves before leaving with a piece of Hawaii. While it might seem like a harmless action, most of us will eventually falter to our just-in-case mentality. After all, its better to be safe than sorry.