Thank you for the lead to the Dublin City Gallery Mauriz.
One good thing about seeing a colour painting is that it is possible to see that the opals were light in colour, so we can definitely discount fire opals.
I had hoped others might have some knowledge of Queen Victoria's other opal tiara.
Here 1854 is also given as the year the painting was done and is based on an extract from the Dictionary of National Biography which is normally accurate.