One of the most meaningful metrics in the market is the gold:silver ratio. It not only provides information about how to trade gold and silver, but offers signals for the stock market as well.
The ratio has meaning because of what the out-performance of one or the other means for the economy and investor sentiment, in general. When gold is outperforming, it normally means investors are risk-averse. When silver is outperforming, investors are risk-taking and the economy has moved into high gear because silver actually has some industrial use, unlike gold. Therefore, we'd expect the gold:silver ratio to be low at market highs and high at market lows.
Besides that, the ratio can offer direction about which you should be buying if you happen to be bullish or bearish on gold or silver. If the ratio is too high, you should buy silver or short gold. If the ratio is too low, you should buy gold or short silver.
I extended the use of the gold:silver ratio to also be something you can actually trade via a pairs trade. This strategy buys gold 50%/shorts silver 50% when the ratio is under 4.9 for GLD:SLV (the actual spot prices of gold and silver are not available on this platform). It then reverses the trade when the ratio > 8 and buys silver 50%/shorts gold 50%. These levels are fairly obvious by investigating a gold:silver ratio chart, which oscillates at regular intervals between those values.
The results are market beating. There was one nasty draw-down that I might consider unacceptable, except that if this were actually trading, I might implement some kind of trailing stop to maximize profits on winning trades and prevent entering before the ratio turns up or down. Even with the draw-down this still does well.