What is this 所 here? This is the sad story of a little piece of gold that wants to shine but can't, so it 哭泣着。

小石头也想：“是金子在哪儿都发光。” 于是他日夜吸收天地之精华，把自己沖涤得闪闪发亮，终究又被脏东西所掩盖。

终究又被脏东西所掩盖。in the end by dirty things 所 covered up 

What should I do with 所 here？？ Just ignore it? Maybe it means 'all covered up'??
http://www.zdic.net/z/1a/xs/6240.htm


  ◎ 所 suǒ
  
  〈助〉
  
  (1) 表示结构 [used before a verb or a V-C construction together with 被 or
  为 to indicate the passive voice]


For translation, you could just ignore it:

In the end covered up by dirty things again.