The novel is set during the New Zealand gold rush, so is it really a surprise that gold is kind of a big fat deal here? In addition to it being somehow key to almost every character's profession, a haul of stolen gold manages to change location and hands multiple times and spark of a lot of bad behavior and crimes, including fraud, theft, and murder.
So, as such, gold kind of gets associated with all of those nasty things—and with the greed that is at the core of those crimes. Crosbie Wells warns Anna about gold's negative effects on human nature when he's giving her the scoop on Lydia's actual intentions toward her:
Here's something you ought to know. There's no charity in a gold town. If it looks like charity, look again. (IV.5.49)
So, in other words, people are only out to make their fortune, and it's every man (or woman) for himself/herself, according to Crosbie.
Of course, the truly dangerous thing about gold is that it can sometimes masquerade as a symbol of hope and happiness. Remember how the gold that circulates through the story ends up stamped with the name of the Aurora mine? Yeah, well, if you've watched the Disney Sleeping Beauty, you know that the name "Aurora" has to do with daybreak, which is a pretty clear symbol of hope and beginning.
Anna herself unconsciously associates gold with "awakening" in her words to Gascoigne when she wakes up in jail and finds gold in her corset:
'Gold,' she whispered. 'It's gold. Up and down the corset-bones, and in the lining, and all the way about.' Her dark eyes were searching his face, pleading with him. 'Gold,' she said. 'I don't know how it got there. It was there when I woke up—sewn in.' (I.7.6)
Given the book's persistent emphasis on how men are coming to the New Zealand goldfields to remake their lives/themselves, it seems significant that so many mentions of gold are paired with references to awakening and dawn, no? No wonder people are so intoxicated by it …
There's another kind of valuable stone in the book that serves as a counterpoint to gold and its significance: pounamu, the greenstone that Te Rau Tauwhare hunts for and carves. Whereas gold and gold digging have to do with commerce in the novel, pounamu is something private and sacred, in Te Rau's view, and a true "treasure" in a way gold isn't:
He did not mention that his skill was as a carver. He had never sold pounamu. He would not sell pounamu. For one could not put a price upon a treasure, just as one could not purchase mana, and one could not make a bargain with a god. Gold was not a treasure—this Tauwhare knew. Gold was like all capital in that it had no memory: its drift was always onward, away from the past. (I.3.163)
Because of his strong opinions about the difference between gold and pounamu, Te Rau gets really upset when Crosbie Wells tries to compare the two, saying they're both just minerals:
'There's no difference,' Wells insisted, reaching for another piece of meat. 'You might not like it—but you have to admit—there's no difference. It's just one mineral or another. One rock or another.'
'No,' Tauwhare said. He looked angry. 'It is not the same.' (IX.4.3-4).
However different they might be in Te Rau's mind, it's worth mentioning that, like gold, pounamu gets associated with violence and murder, since we know it can be used to make deadly weapons. In fact, the book strongly suggests that Te Rau murders Frank Carver with a weapon made out of that sacred stone …