My first thought when reading the article was that tiara mentioned was her meander tiara because it is made of entirely diamonds but you made a good point about her not wearing that tiara in the early years of her marriage. It's more likely that the reporter for the New York Times just didn't have all the facts (think about all of the things royal reporters get wrong nowadays) and the tiara was made of diamonds and another gemstone.
I was fascinated to see the color picture of the necklace. I've never seen it before and if it is truly a color photo and not a colorized black and white photo that would mean the the large gemstones are not aquamarines as is generally assumed. They look to me to be light colored sapphires.
Here are some more pictures of the parure with a better look at the brooch. Also notice the differences between the necklace in the different pics.