Calcite
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

 

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

 

Celestite
Location: Madagascar
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

 

Chrysanthemum Rock 
Location: China
Aragonite crystals in basalt

 

Chrysoprase
Location: Yerilla Australia
Variety of: Chalcedony
Chemical Composition: SiO2 silicon dioxide
Hardness: 6-7
 

 

Cinnabar on calcite 
Location: China
Chemical Composition: HgS, Mercury Sulfide
Hardness: 2 - 2.5

 

Citrine
November Birthstone
Location: Brazil
Variety of: Quartz  
Chemical Composition: SiO2
Hardness: 7

 

Citrine
November Birthstone
Location: Minas Gerais, Brazil
Variety of: Quartz  
Chemical Composition: SiO 2
Hardness: 7

 

 

Crystallized Native Copper
Location: Michigan
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: Adelaide Mine, Dundas Tesinania
 Australia
Chemical Composition: PbCrO4 , Lead Chromate
Hardness: 2.5 - 3

 

Cuprite with malachite
Location: Mashamba West, Shaba Zaire
Chemical Composition: Cu2O, Copper Oxide
Hardness: 3.5 - 4