Calcite
Pure calcium carbonate is colorless, but calcite is often colored by various impurities, including iron, magnesium, manganese, zinc or cobalt. 
Carnelian
Carnelian is a brownish red to orange variety of chalcedony quartz, colored by trace amounts of iron. Darker colors (red-brown to brown) are often referred to by the name Sard.
Cassiterite
Cassiterite is one of the densest gem materials known. It also has a very high refractive index, higher than zircon, sphene and demantoid garnet.
Cat's Eye Apatite
Chatoyancy, is a gemstone that is reminiscent of a cat's eye. It is a choice of many.
Cat's Eye Aquamarine
Cat's eye aquamarine is quite rare and is known for its beautiful blue hues, and the cat eye effect in the middle.
Cat's Eye Diaspore
Zultanite, is a color change gem from Turkey which also goes by the name Diaspore. Cat's eye diaspore is fairly rare.
Color-Change Diaspore
Diaspore, sometimes called Zultanite, originated in Turkey and was recently introduced to the international market.
Color-change Garnet
Color-change Garnet changes color from a brown to a rose pink in dark light due to a mix of spessartite and pyrope garnet. 
Danburite
Danburite was discovered in Danbury, CT, in 1839. It is a gemstone that is quite hard.
Cat's Eye Scapolite
Scapolite is a gemstone that is in column shape. This is because it is actually sodium calcium aluminum silicate and gets its name from the Greek name for "stick".
Cat's Eye Tourmaline
Tourmaline also has a cat's eye effect due to chemical inclusions.
Chalcedony
Chalcedony is the silica mineral quartz that is rather grainy. There are a lot of different colors in this gemstone.
Charoite
Charoite first appeared in 1978. It has lavender and violet hues that are swirly in nature.
Chrome Diopside
Chrome Diopside is a gemstone that is rich forest green because of the amount of chromium found within.
Chrome Tourmaline
Chrome Diopside is colored by chromium and has similarities to Tsavorite garnet and chrome tourmaline. It is green hued.
Color-Change Sapphire
These rare sapphires change their color under certain lighting conditions. Color change sapphires are blue in natural light and purple under incandescent light.
Demantoid Garnet
Demantoid Garnet is extremely rare and comes in shades of light green to emerald. It is usually found is small sizes only.
Dendritic Agate
Dendritic agate is a whitish-gray with fern-like inclusions known as dendrites. The inclusions look like plants but is actually iron or manganese.
Chrysoberyl
Faceted chrysoberyl is not that well know. It has excellent lustre and is a hard stone as well. 
Chrysoberyl Cat's Eye
Cat's eye gemstone is chrysoberyl cat's eye. It is a hard gemstone and has a prominent cat's eye.
Chrysocolla
Chrysocolla is a copper silicate. Chrysocolla has many unusual multi color combinations and also comes in blue or green.
Chrysoprase
Chrysoprase is a chalcedony or cryptocrystalline quartz, which is colored by trace amounts of nickel. Its comes in many shades of green, from light to dark.
Citrine
Citrine is yellow to gold to orange-brown shades of transparent quartz. It is a highly sought after gemstone.
Clinohumite
Clinohumite is a rare mineral in which only three sources of gem-quality material Clinohumite is known to be found in Tajikistan, Siberia and Tanzania.
Coral
Precious coral grows in rocky sea bottoms. Coral comes in a reddish pink color, to a salmon pink and a deep red.
Diamond
Diamond is a transparent crystal of carbon. It is the hardest stone in the world.
Dumortierite Quartz
Dumortierite Quartz is an unusual quartz that contains dumortierite. The inclusions of dumortierite give it a deep blue color that.
