Three natural cristobalite specimens from different localities in Japan were prepared by treating the raw specimens with fluoboric acid. The sample obtained from " Beppu Hakudo " gave rather shar X-ray powder patterns in which many of the lines in the region of back reflection which had first been described in the writer's previous paper were re cognized again. The lattice constants of the sample are larger than those of the standard artificial cristobalite by 0.30%. This discrepancy is

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... able to the simultaneous crystallization of a small amount of tridymite in the sample. The patterns of the other two natural speci mens are rather obscure, but their spacings are in agreement with the standard values within the experimental errors. owing to the difficulty of obtaining good crystals. As reported in his previous paper10), the author was able to obtain good crystals of cristobalite by treating some used silica bricks with fluoboric acid and up to eighty seven powder lines including back reflec tions were clearly observed of them. By treating natural cristo balite rocks in the same way he also obtained good crystals, and in this paper, the results of study on these natural cristobalites will he descrihed.