Chalcedony

Description:

chalcedony (kal-SED-uh-nee) is microcrystalline or cryptocrystalline quartz; hexagonal (trigonal) crystal system.

Appearance:

• semitransparent to opaque 
• virtually all colors

Variety and Trade Names:

milky chalcedony – semi transparent to translucent, nearly colorless or white
• chrysoprase – semi transparent to translucent, light to medium yellowish green (natural color)
• carnelian – semi transparent to translucent,yellow-orange to orangy red, brownish red or brownish orange
• bloodstone (heliotrope) – semitranslucent to opaque, dark green with red to brownish red spots
• prase – translucent to semitranslucent, with a generally darker, less saturated green than in chrysoprase
• agate – chalcedony with curved or angular bands or layers
• landscape agate, picture agate, fortification agate, etc – chalcedony with colored patterns resembling a landscape, etc.
• moss agate or mocha stone – chalcedony with green, black, or brown inclusions that look like moss
• onyx – chalcedony composed of straight, parallel layers of different colors
• sard – semitransparent to translucent, dark brownish red to brown to dark orange (darker and less saturated colors than those in carnelian)
• sardonyx – onyx showing bands of sard colors alternating with either white or black parallel bands
• carnelian onyx – onyx showing bands of carnelian colors alternating with either white or black parallel bands
•  jasper – general variety term applied to semi translucent to opaque chalcedony of any color or combination of colors, except solid black or specially labeled material, such as bloodstone, onyx, etc.
• plasma – semitranslucent to opaque dark green with white or yellowish spots
• chrysocolla chalcedony or chrysocolla quartz – translucent to semitranslucent, intense light blue or blue-green; colored by the copper mineral chrysocolla
• petrified wood, agatized wood, silicified wood – wood that has been entirely replaced by chalcedony; it resembles a patterned jasper
• iris agate – semitransparent to translucent, with iridescent colors (phenomenon best seen on thin slices in transmitted light)
• fire agate – semitranslucent to opaque, with iridescent colors against a brown bodycolor
• dendritic agate – chalcedony with dark inclusions resembling tree branches
• amethystine chalcedony – semitranslucent to opaque purple; also marketed under the name damsonite

Identification:

Refractive Index: 1.577-1.583 (+-.017)
Cause of Color: usually chromium; some are colored by vanadium, or by both chromium and vanadium 
Specific Gravity: 2.72 (+.18,-.05) 
Hardness: 7 1/2 to 8 ; Toughness: poor to good

Stone Information from:

Gem Reference Guide, by GIA, ISBN 0-87311-019-6
Gemstones of the world, Third Edition, by Walter Schumann, ISBN 1-4027-4016-6
https://www.gemdat.org/gem-960.html
https://www.mindat.org/min-960.html