Reader question:

In this sentence – history suggests profits will be far below expectations, possibly making those investments fool's gold – what does "fool's gold" mean?

My comments:

Fool's gold is the common term for iron pyrite, a gold-colored mineral. As pyrite resembles gold in color, it was often mistaken for the real stuff by eager miners during the gold rush in America. As pyrite is relatively¹ worthless, diggers who took it as gold would inevitably² look foolish (not to mention crestfallen) in the end, hence the term.

Metaphorically³, "fool's gold" represents anything that's worthless but is treasured by the gullible⁴. In the opening question, "fool's gold" means those investments might not be smart money, or unwise as they were driven by unrealistically high expectations for profit.

Here are a few media examples:

1. All hype aside, oil shale⁵ is the poorest of the fossil fuels, containing far less energy than crude oil, much less even than hog⁶ manure⁷, peat moss⁸ or Cap'n Crunch⁹. A meager¹⁰ amount of energy, tightly bound up in an enormous volume of rock, oil shale seems destined¹¹ to remain an elusive¹² bonanza¹³, the petroleum¹⁴ equivalent of fool’s gold.

- Oil shale may be fool's gold (Denver Post, December 17, 2005)

2. "I think it's good to be skeptical," he said, after a pause for reflection. "There's a lot of fool's gold in the world, but we all know there's real gold as well and that's why I say look into the teachings, look into what Maharishi's been saying for 50 years and don't go by rumors¹⁵. Look into it and read some of these books and attack it from any angle and it will hold up."

- Peace Man (The Guardian¹⁶, December 10, 2003)

3. A new study says Americans have fewer friends than ever – but what if we're enjoying more solitude¹⁷ and intimacy¹⁸?

...A writer in Albany, N.Y., named Daniel Nester says when he relocated upstate from Brooklyn he chose to bring little of his old life with him beyond his wife and the contents of his apartment – no one from his former circle of friends. "I'm currently not interviewing for new buddies¹⁹. I've downsized my circle of friends to almost nil," he says. "I have one friend. I used to have 30." ...Nester would rather be friendless than in the companionship of what he calls "fool's gold" friends.