quartzite
A hard metamorphic rock composed mainly of quartz, used in construction and industry.
Quartzite is a very hard rock.
この地域では良質な珪岩が採れる。
High-quality quartzite is mined in this region.
珪 (けい, 'silicon') + 岩 (がん, 'rock'). The kanji 珪 is used in geological terms for silica-related rocks and minerals.