Can we choose an opal for engagement ring or wedding bands?

Absolutely!

Opal is a beautiful and unique gemstone that can make a stunning engagement ring and wedding bands. Also Opal has associated with various meanings and symbolism throughout history. In ancient time, opal was believed to bring good luck and protect against evil. 

And due to its uniqueness, it has also been associated with one and the only one love. You cannot find any exact same opal stone in the world, like you cannot find exact same person in the world, therefore using opal in engagement ring or wedding bands has a perfect meaning.