Comparative form of 'coaly'; more like coal in appearance, color, or composition.
From 'coaly' (coal + -y, meaning resembling coal) plus the comparative suffix '-er'. Follows standard English adjective comparison patterns.
When a geologist describes rock as 'coalier,' they're using a word that's less common than 'more coaly,' but it's the kind of technical jargon that appears in mining reports and geological surveys.
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