Veronika thank you for the picture, I'm sure it is the tiara worn as necklace. The distintive shape of the larger and smaller motifs is clearly visible in your picture, also the setting of the pearls, it seems only secured from the back. It matches in every detail the design of the tiara. Plus, if we look careful to the beautiful HQ of the tiara you posted above we can see that the frame has rigid metal spikes to secure each of the several elements of the jewel, it must be very flexible in order to wore as necklace.
And if Countess Raine had two matching pieces of these jewels (tiara and necklace) I bet she would have worn them together for greater efect, and there are only pictures of her wearing it as one of the versions separately .
At this event in 1998 she is wearing her long diamond earrings she often wears. If I am not mistaken she was no longer Countess Spencer by this time, so she had no longer acess to the Spencer pendant diamond earrings.