darkle

verb [vi. 不及物动词]潜藏在暗中，呈显黑色，变暗

If the silvered jewelry do not keep in a sealed environment or wipe it oftenly, it will caused the the color and luster become darkle.
如果镀银首饰不放在密封的环境中保存或不经常擦拭，就会导致其颜色和光泽变暗。