No, you are not completely off the mark, but not quite there.

The classic prospecting tool is the gold pan. First find color in the stream gravels, then follow the shows to the vein up on a hill. That is why a sampling program starts with stream sediment sampling. Every little drainage is sampled, the data plotted up, and follow up sampling are done near the anommalies.

However, we don't just rely on the gold or copper values. There are minerals and elements that occur with the metal of interest, and we look for anomalous areas of those as well.

For example, if we are prospecting for a diamond bearing kimberlite deposit, the chances of us finding diamonds in the pan is pretty small, so we look for garnets and some other minerals that typically form with diamonds, but are far more common than the diamonds. But they have to be the right garnets, and they have to be associated with other minerals. Of course, there is a lot of statistical work that go into understanding the associations, especially when looking for elemental anomalies. We'll sample anything that might give us a clue: rocks, soil, moss, plants, even trees.

With gold, we look for the ugly clay rocks that were formed by the alteration of rocks by the hot fluids that depsoit gold. Then we narrow in on the sulfides and other minerals that might contain gold. Or we chase arsnic to find copper.

This is what ER was talking about, but he was using remote sensing to find salts associated with the alteration associated with the gold.

But now to lithium. While emeralds do contain lithium, they don't occur in large enough deposits to use as a source for the metal. There are other lithium minerals that occur in granites (locally concentrated in what are known as pegmatites) that are concentrated enough to be valuable, but not large enough to waste the time.* Lithium, as it turns out, is best collected from hyper-saline brines from the sediments of desert basins. This Wiki article is useful.

So, while you are correct about pathfinder minerals (and elements) as a concept, in the case of lithium, finding emeralds does not help you find lithium ore. Like I said, just being picky (my intro prof would charge a six-pack for confusing rocks verses minerals verses elements.)

*but this can change. During WWII, many small pegmatites in Colorado, New Mexico, and North Carolina were mined for their mica, beryllium and other elements, but these were small operations.