noun, feminine/ˌpjedɾa anɡuˈlaɾ/plural: piedras angulares
piedra angular is feminine in Spanish.
cornerstone (a stone forming the base at the corner of a building); cornerstone (that which is prominent, fundamental, noteworthy, or central)
“Aquellos años marcaron el debut de los títulos originales, que se convirtieron instantáneamente en piedras angulares del género.” — Those years marked the debut of the original titles, which instantly became cornerstones of the genre.