This is the 3812th most frequent Japanese word.
まる
"まる" means "circle" or "round," and can also signify "correct" or "OK" in contexts like marking answers.
Here, "まる" ("丸", "circle") is used metaphorically in "まるで" to mean "completely" or "entirely", emphasizing the moon-like shape.
この絵の顔はまるで月のような形をしています。
The face in this picture is shaped completely like a moon.
In this sentence, "まる" ("丸", "circle") is part of "まるっきり", meaning "entirely" or "completely", indicating comprehensive engagement in the activity of working.
彼は朝から晩までまるっきり働いています。
He is working entirely from morning till evening.
Here, "丸" ("circle") is used as an adjective "丸い" to describe the round shape of the stones.