I need to identify the figure of speech in this saying:"thy mind is an opal." I think it is a metaphor. Am I correct?

Yes - that's a metaphor. Good!

I need to identify the figure of speech in this saying "good mistress accost, I desire better acquaintance" I think it's a pun am I right?

Good mistress is an example of a malapropism not a pun

Yes, you are correct! The figure of speech used in the saying "thy mind is an opal" is a metaphor. A metaphor is a figure of speech in which one thing is equated or compared to another, even though they are not literally the same. In this case, the mind is being compared to an opal, suggesting that it is colorful, vibrant, and perhaps reflective of different thoughts and emotions, like an opal gemstone. To identify such figures of speech, it is helpful to understand the meaning of different literary devices and analyze the words used in a particular expression or sentence.