The Hagi yunomi (teacup) on my desk is stained from years of use, and some might even say it looks a bit shabby. I prefer to call it full of aji (flavor), the way a pottery connoisseur describes an item with character and patina.
A Hagi yunomi is famous for going through seven stages of change and deepening in aji as more green tea is poured into it. That's how it is with most stoneware yunomi, unlike porcelain that really never changes even after the passing of centuries -- how dull.
A great joy of collecting and using Japanese stoneware pottery is to notice these subtle changes in color tones and crackles that form on a piece. Nowhere is such change easier to find than on the containers used to enjoy steeped green tea, our healthy and adored ocha.