agate, onyx
瑪 centers on banded, decorative stones: agate and onyx. Both are types of chalcedony used in jewelry and ornamentation.
瑪 combines 王 (jade/jewel) with 馬, which likely contributes the sound. The character was created to name specific ornamental stones.
The jewel radical 王 next to a horse 馬: imagine a horse adorned with agate and onyx beads, making it a prized decorative animal.
For メ, picture a horse wearing a medallion made of agate: medallion -> メ, and the jewel-like stone hangs from the horse.
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