Gem silica is a rare form of silica that is most often found coincident with copper deposits in a small number of localities around the world including the U.S., Mexico and Peru. Silica is a clear mineral in most instances but when colored by the inclusion of copper it can take on the most beautiful hues of blue. It is very hard/durable material and can range from blue to blue/green in color and may have sprays of malachite or other minerals.
The most prized (gem quality) material came from small deposits found around copper mines in Arizona (Ray Mine and the Miami/Inspiration Mine) but unfortunately these mines have produced no material since the 1970s. The gem silica I use came from older collections or from dealers that have horded stock that was purchased many years ago. This material is considered to be the best of the best in terms of color and I will typically cut away all inclusions and imperfections unless they add something to the look of the stone. Gem silica is some of the most beautiful blue material found in nature and often sells for upwards of $250/ct.
Other finds include what is called Parrot Wing silica from Mexico that includes a pistachio nut green color as well as a light Caribbean ocean blue. Recently, some new material has come out of Peru from what is called, the Lily mine. Occasionally, I will find some of the prettiest light blue material that has what some might call a Paraiba Blue color named after a region in Brazil where a rare form of blue tourmaline is found. Beware of low-cost simulants of dyed blue chalcedony.