Topaz, Tanzanite and Zircon are all three classified as semi-precious stones as they are affordable to the average layman. But there is, what is called Precious Topaz which fetches higher prices and they are a good rich blue.

BLUE TOPAZ  Topaz can also be found in a variety of colors which includes violet, clear, yellow and the most popular one which is the blue topaz. For a December birthstone you will look for a blue stone, which is not often found in nature. But the color is enhanced by radiation and heat treatment. You will find these stones in engagement rings, men's rings, contemporary and stylish rings as well as necklaces and earrings.
Topazes are known to possess mystical powers too.
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TANZANITES These are rarely blue in the natural state,  but they too can be color-enhanced to a very attractive blue. They were first discovered in Tanzania in Africa, and fetch good prices.
ZIRCON are normally found as brown stones but you do come across some good yellow stones too. They are slightly radio-active, so many avoid using them. I have no qualms about their use as the redioactivoty is ever so small. They are no longer as cheap as they were. They are very affordable in spite of this.