In a gemmy manner; in a way that is bright, sparkly, or jewel-like; with gem-like qualities.
From gemmy (adjective, meaning gem-like or full of gems) + -ly (adverb suffix). Gemmy itself comes from gem plus the adjective-forming suffix -y, creating a chain of derivation from the Latin gemma.
Most adverbs formed from gemmy would be quite rare—you might never see someone actually write 'she gemmily sparkled'—but the fact that the word can be constructed shows how English lets us build new words following predictable patterns.
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