Oh Sue, I wish!
Unfortunately it doesn't have the depth or intensity of the milky-ness of opalescent.
Its more like the effect you get on the bubbles children blow with detergent, or the iridescence on puddles of water on the ground where someone has spilt petrol or engine oil! It's like a very subtle patina, not unlike the iridescence of carnival but so much less intense. I thought it was just a reflection at first, but its not. It also appears to be on the outside as well
The vase has been well used and there is watermarking inside. Also I've just noticed whilst handling it, bears a raised mark on the bottom I cant decipher but I presume its Czechoslovakia! I'm no chemist but I wonder if its a reaction to something in the environment or how its been stored? If perhaps it was exposed to higher than normal levels of sulphur? or other natural occurring minerals.