Cat's Eye Topaz earns attention when light begins to travel across or through it. Collectors who care about optical performance often come to it for different reasons, but identification still comes back to blue body colour, chatoyancy optical effects, and 8 Mohs hardness. The effect is best judged in motion, since a static view rarely shows the full character of the stone.

Among jewellery stones, Champagne Topaz stands out for combining appearance with notable wear resistance. Jewellers who need dependable wear resistance often come to it for different reasons, but identification still comes back to blue body colour, 8 Mohs hardness, and vitreous lustre. The secondary champagne note often matters more than people expect when separating it from lookalikes.

Cherry quartz in commerce is commonly glass or treated quartz-like material sold as a decorative stone.

Chevron amethyst is an amethyst-quartz banded variety characterized by angular purple-white zoning.

Chrome diopside is the vivid green chromium-bearing gem variety of diopside.

Chrome garnet is a trade term for chromium-bearing green garnets, often pyrope-rich material.

Chrysanthemum stone is a patterned ornamental rock with white radiating mineral growths in dark matrix.

Chrysoberyl is a very hard beryllium aluminum oxide gemstone species that includes alexandrite and cat-eye varieties.

Chrysocolla is a hydrated copper silicate gemstone material, frequently occurring as massive blue-green aggregates.

Chrysoprase is a nickel-coloured green chalcedony variety of quartz.

Citrine is the yellow variety of quartz, commonly transparent with vitreous lustre.

Clear quartz (rock crystal) is colorless transparent quartz with trigonal crystal symmetry and vitreous lustre.

Clinozoisite is a monoclinic epidote-group calcium aluminum sorosilicate with limited gem use.

Copper turquoise is a trade material combining turquoise with metallic copper effect components, often stabilized or reconstructed.

Gem coral is an organic biogenic material composed of coral skeleton polished for ornamental use.

Crazy lace agate is a Mexican banded agate known for dense swirling lace-like patterning.

Crystal Opal earns attention when light begins to travel across or through it. That combination of multicolour body colour, opalescence, play of colour, iridescence optical effects, and 5.5 Mohs hardness is what usually makes it recognisable in practice. The effect is best judged in motion, since a static view rarely shows the full character of the stone.

Cymophane is the traditional mineralogical name for cat's-eye chrysoberyl.

Dalmatian jasper is a trade name for spotted ornamental rock, not a strict quartz jasper species.

Danburite is a calcium borosilicate gemstone often seen as a colorless faceted stone.

Demantoid is the green gem variety of andradite garnet, prized for high dispersion and brilliance.

Dendritic agate is chalcedony containing tree-like manganese or iron dendritic inclusions.

No gemstone has a stronger reputation for durability than diamond, and that reputation is grounded in crystal structure rather than marketing. Its exceptional hardness, high lustre, and ability to return crisp light make it central to both jewellery and industrial applications.

Dioptase is a bright green copper cyclosilicate mineral prized mainly as a collector crystal and occasional gem.