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). 
Learn more about calcite!

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
Learn more about chrysoprase!

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

Judy's Citrine
November Birthstone
Location: Brazil
Approx. 12" long and 5" at it's widest
Yellow to gold to brownish gold quartz.
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
Usually controlled by environmental laws

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