I do think that they are rubies because of the way there are several stones put together to make what looks like a bigger stone. If they were garnets, spinels, or some other stone then the wouldn't have had to do that.

Since I wrote the post I also found a picture of Crown Princess Khaleeda wearing the tiara but it doesn't help us in regards to getting a better look at the tiara.



Also here's the full version of Permaisuri Raja Zarith Sofiah wearing the parure