agate, onyx
瑙 centers on banded gemstones, specifically agate and onyx, which are varieties of chalcedony with layered patterns.
瑙 is a phono-semantic compound: 王 (jade/gem) indicates the meaning, while the right side (originally 匘 or a similar component) likely contributed the sound ノウ. The exact historical development of the right side is uncertain.
The left side 王 (jade) tells you it's a gemstone. The right side looks like a brain (腦) with its convoluted folds, like the swirling bands inside agate and onyx.
For ノウ, imagine a gemologist saying 'Now, this is a fine agate!' as they examine the stone: now -> ノウ.