high quality porcelain
瓷 centers on fine, high-quality porcelain, a refined ceramic material.
瓷 combines 瓦 (earthenware) with 次, which likely contributes the sound. The character originally referred to a specific type of fine pottery.
次 (next) over 瓦 (tile) suggests the next level of pottery: fine porcelain, superior to ordinary earthenware.
For シ, imagine a sheet of high-quality porcelain: sheet -> シ, smooth and refined like the porcelain itself.