It is against Hawai'i law to take sand from Hawai'i. Shells are permitted but not for commercial use, but we ask you NOT to take them as our beaches don't have an abundance of shells due to the reefs surrounding the islands.
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Tip: Pineapples are on the USDA list of agricultural items permitted to leave Hawaii. As long as there are no bugs, soft spots or holes you can take them home to the mainland. It makes for a perfect souvenir.
And that's a very risky endeavor, as taking sand from any beach in Hawaii is punishable by fines upwards of $100,000. This includes Papakolea Beach, also known as Green Sands Beach, and Punalu'u Beach, which is famous for its black volcanic sands.
You shouldn't have any problem as long as they are clean and don't smell. If you collected these yourself from a beach within the EU they aren't likely to be from endangered species, which would be the only probable reason for any restriction.
In certain countries, removing seashells from the beach or coral is actually illegal. This is certainly true in the UK, where under the Coastal Protection Act of 1949, it is illegal to take any kind of natural materials from public beaches. Anyone caught doing so by authorities could be fined up to £1,000.