There are two primary figures of speech which are used to compare one entity to another.  One of them is called "simile"; the other is called "metaphor."

A simile normally uses the word "as" or "like" to make the comparison, such as: "He ran as fast as a jackrabbit"; "His face was dirty as a coal miners' at the end of the shift"; "Her skin was like satin."

A "metaphor" does not say that a thing is LIKE another thing.  Rather, it states that one thing IS the other.  For example:  "My heart is a wanderer."  "My stomach is Jello." 

Your quote, "thy mind is a very opal," is a metaphor.  Now, think of the characteristics of an opal, and try to think of a person's mind as being the same.