This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Earth Crest: It's at the end of a pipe in the Steamwood Forest. It
allows you access to the wind crest.
Water Crest 1: Look for this crest in the Grillin' Resivoir. It refills
the resivoir when activated.
Water Crest 2: There's another crest just outside town, surrounded by a
little lake. Using this crest drains the lake, allowing you to get the
treasure box inside.
Fire Crest: This is also inside the Grillin' Resivoir, in the cave which
becomes available after the resivoir is refilled.
Wind Crest: It's just beyond the Earth Crest. Note that it can only be
activated when it's raining. Activating it takes you to the place where
the Sky Scroll is located, but watch out: As Jon warns you, once this is
done, you can't return!
Sky Crest: (It would be unfair to reveal what this crest does...)