Pencil, tombstone

It occurred to me today that the same Latin word lapis (stone) became lápis (Portuguese), lapis (Italian) and lápiz (Spanish) meaning pencil, and lápide (Portuguese), lapide (Italian) and lápida (Spanish) meaning tombstone. I understand how Portuguese and Italian got lápide/lapide from Latin lapis. Lápide coincides with the Latin ablative of lapis: lapide, but why Spanish has lápida puzzles me.