Rare metals are two unique types of Meteos, Time and Soul, appearing only in the DS version of Meteos. Occasionally, one of these two Meteos types will drop down to the bottom of the screen, destroying all blocks below it. This can be annoying, particularly when attempting to Screen Clear, as one must rejoin the blocks together. Nevertheless, certain fusions require Rare Metals, so launching them offscreen is crucial to unlocking everything.
It is not as hard to get these Meteos as one may think at first. You do not need to turn them into rockets, as that actually wastes them. What has to be done is turn Meteos into rockets by moving them below the Rare Metal (skip if there already are three ready-to-launch Meteos below it) and launch them. This will send the Rare Metal into space and you will obtain it.
In the Fusion Room, Time Meteos are described as the "Ruler of Start and End". Once you have seen 9 different Star Trip endings, you may create 1 Time Meteo by fusing 500 Air, 500 Soil, 500 Iron, 500 Herb and 200 Dark.
In the Fusion Room, Soul Meteos are described as "The Root of all Life". Once you have seen 9 different Star Trip endings, you may create 1 Soul Meteo by fusing 500 Fire, 500 H20, 500 Zap, 500 Zoo and 200 Glow.