An Opal Cocktail? Um, yes please!

Well, how about a little cocktail or two to get into the spirit of October,
and to honour the glorious birthstone of the month,
that multifarious Opal?

This is an oldie, but a goodie. 
And citrus flavoured cocktails always seem to be popular,
whatever the season.
Which reminds me, we're out of Cointreau. Again...