Flint

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Old English; related to Middle Dutch vlint and Old High German flins .

wiktionary

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From Middle English flynt, flint, from Old English flint, from Proto-West Germanic *flint, from Proto-Germanic *flintaz (compare Dutch vlint, flint(“flint, cobblestone”), German Flins, Flint(“flint, pebble”), Danish flint(“flint”)), from Proto-Indo-European *splind-(“to split, cleave”) (compare Irish slinn(“slate, shingle”), Ancient Greek πλίνθος(plínthos)), from *(s)plei-(“to split”). More at split.

etymonline

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flint (n.)

Old English flint "flint; a type of rock noted for hardness and for giving off sparks when struck," from Proto-Germanic *flintaz (source also of Middle Dutch vlint, Old High German flins, Danish flint), from PIE *splind- "to split, cleave," from root *(s)plei- "to splice, split" (source also of Greek plinthos "brick, tile," Old Irish slind "brick"), perhaps a variant of *spel- (1) "to split, break off." Transferred senses (hardness, etc.) were in Old English.