Natural White Topaz is not a substitute of a substitute. Natural White Topaz is a substitute of Natural White Sapphire (which is another natural gemstone). Natural White Sapphire is a variety of Corundum and Gemstones like Ruby, Yellow Sapphire, Blue Sapphire, Green Sapphire, White Sapphire etc. comes into this family. There are 100s of different Natural Gemstones.
Natural White Zircon is the best substitute for a Natural White Diamond.