Shell | Gemopedia

Shells were one of the first items used for ornamentation, and for good reason. Unique, one-of-a-kind shells have their own allure and appealing, natural beauty. Formed as the hard outer covering of many mollusks, shell is grown in layers by cells in the mantle, a skin-like tissue in the mollusk's body wall. As there are many varieties of mollusk, shells come in an endless array of shapes, sizes, and colors.
Colors
White, Gray, Brown, Yellow, Orange, Pink

Shell Classification

Common Name

Shell

Species

NA

Variety

Colors

White, Gray, Brown, Yellow, Orange, Pink

Alternate Names

Gemstone Groups

Key Separations

Appearance, RI, birefringence blink and magnification.

Comments

Misnomers: Chinese cat's-eye, shell cat's-eye Helmut shell are usually carved as cameos, generally layered in two colors, white and brown or white and orangeConch shell: pink to orangeOperculum: white, green and brown with eye-like appearance

Shell Optical Properties

Transparency

Translucent - Opaque

Refractive Index

1.53-1.685

Birefringence

0.155

Optic Character

Optic Sign

Polariscope Reaction

Aggregate (AGG)

Fluorescence

SWUV: Variable
LWUV: Variable

CCF Reaction

Pleochroism

None

Dispersion

Comments

Shell Chemistry & Crystallography

Chemical Name

calcium carbonate, conchiolin and water

Chemical Formula

CaCO3 with H2O and other organic materials

Synthesis

Crystal System

NA

Classification

Organic

Nature

Natural

Crystallinity

Non-Crystalline

Comments

Shell Characteristic Physical Properties

Hardness

3.5

Streak

Specific Gravity

2.7-2.89 Typical:2.86

Toughness

Fair

Inclusions

Shell cameos will show concave backs due to structure of the shell. Shells might show orient and will have a parallel growth structure.Helmut shell cameos will have a straight irregular fibrous structure and concave back.Operculum is the eye-like marking on the front and spiral growth on the back of a shell.Conch shells will have a flame-like structure.

Luster

Greasy

Stability

Fracture

Uneven, Splintery

Cleavage

None

Comments

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