I think that a lot of times the diamonds that are used to accent other gem stones in earrings or a pendant aren't that good of a quality. White sapphire or topaz will work just as well and for less cost.
I think I agree with this. As I said in the OP, I really couldn't tell the difference between the halos on the earrings I was comparing. When I got the sapphires home I stuck them in and had a pair of diamond studs that my parents gave me years ago in my second piercing and it was pretty clear that the sapphires were more sparkly than the larger but not awesome-quality diamonds. I have several other pieces that were gifts (pendants, mostly) with smaller diamonds that seem to sparkle a lot only in very bright sunlight.
It really made me think about whether investing in diamonds is worth it in some cases as I begin to move into a stage of my life where I can buy my own jewelry. I do like the durability of diamonds, but moissanite is another good option for super-durable pieces.