tortoise shell
玳 refers specifically to tortoise shell, especially the ornate material used in traditional crafts.
玳 is a phono-semantic compound: the 王 (jewel) radical indicates a precious material, and 代 (タイ) gives the sound. It was created specifically for the word 玳瑁 (たいまい, hawksbill turtle shell).
The 王 (jewel) radical on the left marks it as a precious material, and the right side 代 suggests its sound. Picture a beautifully patterned tortoise shell used like a jewel.
For タイ, imagine tying a decorative tortoise shell ornament with a string: tie -> タイ, and the shell is a treasured jewel.
hawksbill turtle (Eretmochelys imbricata)