noun
ferruginous quartz
Technical term in mineralogy for quartz containing iron impurities, often with a reddish or brownish color.
この標本は鉄石英で、鉄分によって赤みを帯びている。
This specimen is ferruginous quartz, and it has a reddish tint due to iron content.
Compound of 鉄 (てつ, 'iron') and 石英 (せきえい, 'quartz').