So Shane placed the piece on one of the pinnacles of volcanic rock protruding out of the rim of the great crater which makes up the island ... and, hopefully, the cycle of bad luck was broken.
Tuhua, which lies 35km north of Tauranga Harbour, was highly prized by Maori because the of the way the glass-like obsidian, created by the island's numerous volcanic eruptions, could be split to create razor-sharp cutting edges.
The power of the obsidian rock is illustrated by a legend Reon told us, about the two lakes which lie within the crater, one coloured green (because of algae) and the other black (due to fine particles of rock). A long time ago, he said, pounamu, the greenstone also greatly valued by Maori, and tuhua, the obsidian, fought a bloody battle for supremacy. Tuhua won, and chased pounamu all the way to the South Island, but the signs of that battle remain today.
"The black lake is called the blood of obsidian. The green lake is the blood of greenstone and it is the larger of the two because greenstone lost the most blood."
Obsidian from Tuhua was widely traded among Maori and has been found as far north as the Kermadec Islands and as far south as Tiwai Point at the bottom of the South Island. "It made my ancestors very rich," said Reon with a smile.
To protect this wealth the inhabitants built a series of powerful pa, like Taumoe which once occupied the area where we were standing, protected by the steep walls of the crater on one side and the sheer cliffs rising from the sea on the other. "Taumoe was impregnable," said Reon. "It was never defeated."
I could readily understand the problems facing enemy invaders. Getting here had involved a steep climb from our landing place at Opo Bay up the 700m-high crater wall, then more than an hour's walk on a track running along the crater rim, which in most places was only 1-2m wide, with sheer drops on either side.
And finally we had to overcome the Devil's Staircase, a nerve-wracking exercise requiring us to don safety harnesses which were clipped to a steel rope, then clamber up and down a series of rock steps, at one point with the aid of a steel ladder.
It was no great surprise when Reon told us, "many warriors died trying to come up here". Pointing to the stacks of boulders lying on the beach several hundred metres below he added, "Their bones are said to lie still beneath those rocks".
At several points on our route, but especially at Taumoe, we came across deposits of obsidian, mostly black but in one area caramel-coloured, and Reon explained that there were actually nine different varieties on the island.
Once Shane picked up a sliver which was quite literally razor-edged and commented, "You could shave yourself with this. Who needs a shave? Jim?" Reluctantly I declined.
But I did wonder why, if the island was so rich, no one lived here today. "Ah," said Reon, "when the Pakeha came with metal no one wanted the tuhua any more. Suddenly it was worthless."
The population of the island gradually declined until "in 1901 the last person left for Waihi Beach" after which the place was largely deserted and overun with pigs, cats and rats.
But Tuhua got a new lease of life in the 1960s when a partnership between the Maori owners and the Department of Conservation saw the island cleared of pests and pine trees and turned into a wildlife sanctuary. Later, part of its coast was made a marine reserve.
The results are obvious. The island is now thickly forested with native trees and the birdlife is prolific. As we walked around we were constantly serenaded by bellbirds and tuis. Cheeky little robins and fantails came to check us out. The distinctive cry of stitchbirds echoed through the trees.
At one point a kaka swooped overhead shouting abuse. "He always does that when we enter his territory," said Shane. And, sure enough, on the way back we got another telling off.
The island also provides a refuge for rarities like kiwi and tuatara, and on the black lake is a flock of pateke, the endangered native teal.
Alongside the track we came across a large rock which Leon persuaded the only female member of our group, Kristen Ude, a visitor from California, to sit on. "This," he said, "is Te Toka Hapu, pregnant rock, which our women used to visit when they wished to have a baby."
Kristen put on a worried look. "How am I going to explain this to my boyfriend?"
Tuhua is also home to an ancient pohutukawa forest ruled over by a mighty tree, known simply as Te Rangatira, the chief, which resides a 10-minute walk inland from Opo Bay.
When Reon led us into the presence of this giant, he performed a respectful karakia and then did a hongi with the great trunk. Could he sense its mauri? "I can always feel its mauri," he replied. "I believe it has stood here 2000 years and it knows ..."
I felt a sense of awe, much as I do when visiting Tane Mahuta, the great kauri in Waipoua Forest, and it felt perfectly natural to hongi with a living thing which has far outlasted the lifespan of any mere human.
A sign nearby said the tree was 4-8m in diameter which seemed to allow for a rather large margin of error. But Shane said he had measured the trunk using a strop - "we didn't have a tape that was big enough" - and it was more than 11m in diameter.
It is said to be the second largest pohutukawa in New Zealand, second only to Te Waha o Rerekohu, the great tree at Te Araroa, near East Cape.
Of course the reason Te Rangitira and its neighbours have survived the ravages of human settlement is that the island remains fairly inaccessible.
I had been wanting to visit Tuhua for many years, ever since I discovered that my great-great-grandfather had called at the island several times in the 1830s on HMS Buffalo. The ship used to sail out to Australia to drop off convicts and troops, cross the Tasman and call at Tuhua to pick up Maori labourers, take them up to Tairua to fell kauri growing on the banks of the river, then sail back to England with a cargo of spars.
When the Tauranga Gamefishing Club had a famous base on leased land at Opo Bay there was a constant parade of boats. But since the club moved to a new base on the mainland the island has not been an easy place to get to. There is no jetty and though the buildings used by the club still stand beside the shore at Opo Bay they are mostly dilapidated and due to be be removed later this year.
A few cabins are habitable - just - and accommodate an occasional caretaker from Te Whanau o Tauwhao and students from Bay of Plenty Polytechnic's marine studies course who monitor the marine reserve.
The iwi is discussing plans to build a marae on one of the more accessible pa sites to provide somewhere for visitors to stay.
For the time being, though, the only people who come here are a few boaties, who are allowed to land if they pay a $5 fee to the caretaker, and passengers on the helicopter trips which Aerius Helicopters has been running since the iwi granted Shane exclusive landing rights earlier this year.
These flights are a great way to see Tuhua, partly because the journey takes only 15 minutes, but also because it provides a wonderful view of the island and, along the way, Tauranga Harbour, its entrance guarded by Mauao (the Mount) and other islands beyond.
Prominent among these are the rocky pinnacles of the Aldermen Islands, named by Captain James Cook in 1769 because their portly shapes reminded him of a group of city councillors, presumably inspiring him to call their even fatter neighbour Mayor Island (though some suggest it was because Lord Mayor's Day was due to be held in London a few days later)
From the air you get a clear picture of the 15km-wide crater which makes up the island, with the green and black lakes sitting on its forested floor, and when the helicopter hovers off the cliffs you can even see the layers of volcanic rock left by successive eruptions.
It is a remarkable place, made unique by the geological forces which have shaped it, and made even more special by its status as a sanctuary.
As I walked the tracks around Tuhua it was easy to imagine that - apart from the disappearance of its Maori community - the island was much the same as when my great-great-grandfather came here 175 years ago.
When I got back, my wife asked if I had brought back a piece of obsidian to add to the collection of rocks in our garden from places we have visited. No, I said. There was plenty of obsidian there for the taking but it didn't seem right.
Jim Eagles visited Tuhua as guest of Aerius Helicopters.
