In the coldest flint there is hot fire.

An outwardly cold or unyielding person may still be capable of strong feeling. Their manner does not reveal everything they hold within.

Flint can produce a spark despite its cold, hard surface. Likewise, warmth or intensity may become apparent only when circumstances draw it out.

Use it to caution against judging someone solely by a reserved or severe manner, or to reflect on an unexpected display of feeling or determination.