第 4 集: Ruri 利用磁力加速藍寶石源分析，Yoko 則研究螢石。

Seeking sapphires, Ruri searches sands. Nagi explains that river sand recaps the geology of all upstream drainages. Examining sand from each tributary and recording both positive and negative results will eventually lead to the source of the sapphires. But the method requires hours of work on the binocular microscope. Ruri finds a shortcut by running a magnet over the sample to remove the magnetic fraction. Yoko works on her fluorites, which may contain tungsten. Ruri's friends begin to wonder how she's spending her free time. (Source: Wikipedia)

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