All the things she had been believing in until then had come to mean nothing, as gold turns into coal.

come 是动词，意思是"来、到达"，在这里表示"变得"或"达到某种状态"。 to 在这里是介词，表示"达到某种程度或状态"，与 come 搭配使用表示"最终变得"。 come to mean nothing 这个短语表示"最终变得毫无意义"，强调从有意义到无意义的转变过程。