HM planned to wear the powder blue gown with the diamond riviere, diamond earrings and part of the floral tiara. Then...on the morning of her birthday her loving husband presents her with the turquoise jewellry. She doesn't have time to change her choice of gown but she dearly wants to wear the Prince Consort's gift, resulting in the the necklace not being shown to its best advantage.
I agree with Arthur that the necklace isn't that bad. It could look quite nice with a more modern style gown with plain fabric and simple lines.