There is not much doubt that the best alternative to Diamond for the price is CZ. The simulant Moissanite is probably even better but at nearly 100x the price, it takes unique scenarios to justify this difference.  Notice the high dispersion rates for CZ and Moissanite.  That is why you might notice a “glitzy” look to these gems. This “fire” is sought after in Diamond, but sometimes can be telltale of a synthetic.

Amongst the natural alternatives, I like Zircon for everything but rings. However, it is extremely difficult to find in any consistency.  White and blue Zircon require heat treatment to produce the color and since blue is much more popular, you don’t see much white Zircon in the market. In light of this, I would recommend white Sapphire as an excellent natural alternative. Its hardness makes it useful in rings and its optical properties are satisfactory for many applications. For cost sensitive situations, you’ll find natural white Topaz is an excellent value. Cutting becomes imperative in this gem to bring out its best sparkle, but customers that dislike the sound of CZ or any synthetic may appreciate the compromise with Topaz.  

Cutting is critical to the brilliance of all gems, but especially the gems with lower refractive indices. Refractive Index is often associated directly with “sparkle”, but in reality it only contributes to it. A “fine cut” gem will always “out sparkle” a poorly cut gem regardless of Refractive Index!