Impression Stone

calcite (KAL-site) is a mineral species; hexagonal (trigonal) crystal system; chemical composition CaCO3
opaque
pale green with brown stripe
onyx marble – banded marble

Identification

Optic Character: DR, uniaxial negative; AGG
Refractive Index: 1.486-1.658
Birefringence: .172
Dispersion: .017
Pleochroism: none to weak
Ultraviolet Fluorescence: variable
Absorption Spectra: any lines seen are due to impurities or dye
Specific Gravity: 2.70 (+-.05)
Cause of Color: various impurities
Polish Luster: vitreous to greasy
Fracture: granular to uneven to splintery
Luster: dull (on aggregates) to subvitreous
Cleavage: perfect in three directions; often obscured in aggregates
Identifying Characteristics: strong doubling in transparent varieties; high spot birefringence on aggregates
Hardness: 3
Toughness: poor in single crystals, fair to good in aggregates

Enhancements

Method: dyeing

Effect: adds color to marble varieties
Prevalence: common
Detection: magnification, spectrum, color filter
Stability: dyes may fade
Special Care Instructions: avoid solvents, heat, and prolonged exposure to strong light

Method: paraffin or plastic impregnation

Effect: improves the appearance of the polish
Prevalence: common
Detection: hot point
Stability: paraffin may discolor
Special Care Instructions: avoid solvents and heat