You have probably not heard of this gem before. Gem silica is a rare variety of greenish blue chalcedony. Small amounts are currently produced in Arizona and a few other locations. It is the rarest and the most expensive variety of chalcedony.
Gem silica receives its wonderful color from small amounts of included chrysocolla or copper compounds. It is found in arid areas, in the rocks above copper deposits.
Gem silica has a Mohs hardness of 7 and is a very tough stone. That makes it perfect for use in almost any kind of jewelry.
You will probably never see gem silica in a mall jewelry store or in a department store. Gem silica is primarily used by high-end jewelry designers who are making one-of-a-kind jewelry items. At the present time (late 2015) it does not have an astronomical price. The 1.5-carat trillion-cut stone in the photo had a retail price of $179.
Tanzanite is a violetish blue gem that is only found in commercial amounts in the African nation of Tanzania. Tiffany and Company selected the name to honor the country where it is produced.
Tanzanite is a relatively new gem, first discovered in the 1960s, whereas most other popular gems have been known and used for hundreds, if not thousands, of years. Tanzanite's wonderful blue color and Tiffany's promotion quickly made it an extremely popular gem.
In the United States, tanzanite is the second most popular blue faceted gem after sapphire. It costs much less than sapphire, but is every bit as attractive - some people even prefer tanzanite over sapphire. For those reasons, tanzanite has become a lower-cost alternative to sapphire.
Tanzanite is not as durable as sapphire. Sapphire has a Mohs hardness of 9, while tanzanite has a hardness of about 6.5. Tanzanite is best used in earrings, pendants, and pins, where abrasion and impact are unlikely. If it is used in a ring, bracelet, or other impact-likely item of jewelry, a design that guards the stone should be used.