Harvey told the Herald the main greenstone piece is about 10cm long.
She initially wanted to conceal where it was found, as a way of verifying any claims for the necklace but she had since changed her mind.
"I don't think the wrong person would try and claim it."
It was found at Five Mile Bay.
"We went to put our stuff down and as I walked I kicked sand on it and I was like, hang on, that's greenstone, and picked it up.
"It's quite worn, so it's obviously old or been well loved. We found it on the bank with some stones. It had been pushed up against the bank like someone had put it there to be safe, maybe to go swimming, and just forgotten about it."
Harvey said they took the necklace to the police but there seemed to be little interest so they decided to hold it for return when the owner is identified.
"If we can't find who it may belong to, we're not too sure what we would like to do, but maybe take it to a marae around that area who could probably safe-keep it for the time being.
"It looks very precious."
Harvey said "everyone" in her family had greenstone.
"That's why as soon as we found it, it was wow, this actually has to be given back to the right person.
"All greenstone, I believe, is given as an important or a symbolic gesture. You just don't buy greenstone for yourself and keep it for yourself.
"I just keep thinking maybe it's from someone that's passed on and got sentimental value from a child or a grandparent or something like that."
• If you know who the necklace belongs to, Lydia Harvey can be contacted via Facebook and at 021 901 164