I may have a ridiculous grudge against H. P. Spofford: for an American Gothic class I had to buy a book of hers and it was rather outrageously priced for a collection of short stories, I thought. That said, I think I can approach this story with some claims of neutrality.
The meat of the story is the nameless narrator's adventure searching for the Northwest Passage or the fabled Arctic Sea--the patch of open water some people believed was at or near the North Pole. By "meat," I mean "majority": he is tasked with finding the Passage on page 92 and is rescued on page 101, leaving only 90-1 free of ice.
Stories about amazing adventures in the polar region will always recall Poe for me, though he wasn't the only person writing ice-sea fantasias in the 1900s. And for all that Poe occasionally goes for the breathless, I think Spofford's style of concatenation without conjunction is her own:
I sprang from that block to another; I gained my balance on a third, climbing, shouldering, leaping, struggling, holding with my hands, catching with my feet, crawling, stumbling, tottering, rising high and higher with the mountain ever making underneath...Personally, that sort of breathlessness doesn't do much to illuminate what's really going on. I found myself mostly getting exhausted by her lists of verbs. (I feel the same way sometimes about Walt Whitman's lists of nouns, which start about the same time.)
That said, while the centerpiece of this story is all about nameless narrator's adventures, the frame is worth noting: although he's well off enough, the narrator has this irrational fear of poverty, for which he has put off marriage with his fiancee; and so he agrees when his rich uncle (a "misogynist") promises him early inheritance if he discovers a Northwest Passage (which, notably, his uncle doesn't believe in--that is to say, this seems a bit like a snipe hunt); but when he comes back, the narrator marries his fiancee and promises to feel rich in her affection--except he's still haunted by the idea of the treasure he passed up in the Arctic: the mass of moonstone that gives this story it's title. So for all the story seems like a happy-ended comedy--they get married, no more Arctic adventures--there's a tiny hint of obsession and tragedy at the end.