In chapter two of The Pearl, by John Steinbeck, Kino and his little family are in the canoe as Kino goes diving for pearls. He sees the "great oyster" and senses there may be something special in it. When he arrives back at the surface, Juana urges her husband to open it. This is what he happens:
Kino deftly slipped his knife into the edge of the shell. Through the knife he could feel the muscle tighten hard. He worked the blade lever-wise and the closing muscle parted and the shell fell apart. The lip-like flesh writhed up and then subsided. Kino lifted the flesh, and there it lay, the great pearl, perfect as the moon. It captured the light and refined it and gave it back in silver incandescence. It was as large as a sea-gull's egg. It was the greatest pearl in the world.
The two similes Steinbeck uses in this description are "perfect as the moon" and "as large as a seagull's egg." Each of these comparisons reveal important qualities of Kino's great pearl. First, it is perfectly round (something that is not true of many uncultured pearls), just as a full moon is perfectly round. Second, it is large, the same size as a seagull's egg. Unfortunately, few modern readers will know what that means, specifically, but Kino and the other villagers do. Steinbeck uses these similes to create effective description.