Minerals C

Calcite

Location: Mexico, China, Brazil & More
Chemical Composition: CaCO3, Calcium Carbonate
Hardness is 3 (only on the basal pinacoidal faces, calcite has a hardness of less than 2.5 and can be scratched by a fingernail).

Carnelian 

Alternate August Birthstone
Variety of: Chalcedony
Chemical Composition: SiO2 
Hardness: 6.5 to 7
Location: Madagascar, USA, India, Brazil, Siberia, Germany & more

Cavansite

Location: India, USA, and more
Chemical Composition: Ca(VO)Si4O10(H2O)4, Hydrated Calcium Vanadium Silicate
Hardness: 3 - 4

Celestite

Location: Madagascar, & smaller quantities worldwide
Chemical Composition: SrSO4, Strontium Sulfate
Hardness: 3 - 3.5

Charoite

Location: Russia
Chemical Composition: K(Na, Ca)11(Ba, Sr)Si18O46(OH, F)-nH2O, Hydrated Potassium Sodium Calcium Barium Strontium Silicate Hydroxide Fluoride
Hardness: 5

Chrysoberyl

Location: Brazil, Madagascar, Myanmar, Pakistan, Russia, the United States and Zimbabwe
Chemical Composition: BeAl₂O₄
Hardness: 8.5

Chrysocolla

Location: Peru, Africa, Mexico, USA
Chemical Composition: Cu₂H₂Si₂O₅(OH)₄
Hardness: 2-4
Minor ore of copper

Chrysanthemum Stone 

Location: China
Chemical Composition: Al₂SiO₅
Aragonite crystals in basalt

Chrysoprase

Location: Australia, Brazil, USA
Variety of: Chalcedony
Chemical Composition: SiO2 silicon dioxide
Hardness: 6-7

Cinnabar

Location: Bohemia, Hungary, Serbia, Austria, Spain, California, Texas, Nevada, Europe
Chemical Composition: HgS, Mercury Sulfide
Hardness: 2 - 2.5

Citrine

November Birthstone
Location: Brazil, Namibia, Russia, Madagascar, & more
Variety of: Quartz  
Chemical Composition: SiO2
Hardness: 7

Copper

Location: Michigan, Chile, Peru
Metallic light rose, light orange to red.
Chemical Composition: Cu, Elemental Copper
Hardness: 2.5-3

Coral

Organic Gemstone
Colors: red, pink, white, black, or blue
Chemical composition: CaCO or organic substance
Hardness: 3 - 4

Crocoite

Location: Australia, Russia, Germany, France
Chemical Composition: PbCrO4 , Lead Chromate
Hardness: 2.5 - 3

Cuprite

Location: France, England, Australia, Namibia
Chemical Composition: Cu2O, Copper Oxide
Hardness: 3.5 - 4