I thought I would share these lovely little references from William Shakesheare regarding opals.
This is from Stanza 31 in Shakespeare's sonnet Lovers Complaint.
"The diamond, why, 'twas beautiful and hard,
Whereto his invis'd properties did tend,
The deep-green emerald, in whose fresh regard
Weak sights their sickly radiance do amend;
The heaven-hued sapphire and the opal blend
With objects manifold"
In Twelfth Night, Act II, scene 4, the clown says to Duke Orsino:
"Now, the melancholy god protect thee, and the tailor make thy doublet of changeable taffeta, for thy mind is a very opal."
The thing that is so lovely about the reference to Opal in Twelfth Night is that Shakeshere makes an ingenious metaphor based on the knowledge of the change in colours and appearance of opal as it moved in relation to the incidental light.