4화: 루리는 자석을 이용해 사파이어 원료 분석을 가속화하고 요코는 형석들을 연구한다.

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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