GLD and SLV are closely correlated pairs. On the chart below, you'll see that, for the vast majority of the time, GLD and SLV enjoy a correlation darn near close to 1.0, which basically means that their trajectories are virtually identical the vast majority of the time. However, at various points, that correlation diverges dramatically, particularly at the end of April and the beginning of September, after which their respective trajectories reverted to their historic correlations. These divergencies are profit opportunities.
How do you trade these divergencies? You can either trade them buying and selling the underlyings or use options to do so. If using underlyings, you would buy shares of SLV and sell shares of GLD such that the dollar value of the SLV shares is roughly equal to the dollar value of the GLD shorts at the point at which the shows them to be most divergent (near 0.0) and exit both trades when they return to convergence (at or near 1.0). If using options, there are a variety of ways to trade the divergence, but as a premium seller, I would probably opt for selling short put spreads in SLV with the short put strike at or around the 1 SD (standard deviation) and selling short call spreads in GLD at or around the 1SD at the point of greatest divergence, using the Buying Power Effect to balance the short put side ( SLV ) with the short call side ( GLD ). By trading the pair in this fashion, you will not only get the benefit of options time decay, but also the price movement from the return to convergence that selling pairs gives you.
Currently, I don't think a GLD/SLV pairs trade is necessarily "ripe" for an entry, but it's worth keeping an eye on ... .