Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course

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Dendritic, Moss and Plume Agates

A subvariety of chalcedony or microcrystalline quartz, these agates have in common that they contain mineral inclusions which may be any color, but share a roughly tree-like or branching form (dendron = tree). Those which have a more plant or feather-like appearance have been called moss or sagenitic or plume agates, respectively. All agates take a wonderful polish and are tough enough for most jewelry uses. Designers often take advantage of the intriguing patterns these stones have to offer.

[Dendritic Agates]

In general agate prices are quite modest with most of the price paying for the fashioning rather than the material itself. In the case of these agates, those with particularly distinctive or landscape like patterns or those of especially large size are at a premium. Custom cutting or pieces from collectible locations would be substantially more expensive. Pieces with especially fine patterns can be quite valuable.

[Moss Agates]

[Plume or Sagenitic Agates]
Gemological Data:
Makeup: Silicon dioxide, in cryptocrystalline form
Luster: vitreous
Hardness: 7
Crystal structure: trigonal
Cleavage: none

Fracture: conchoidal

Density: 2.60 (+.10, -.05)
RI: 1.53-1.54
Birefringence: none to .004
Pleochroism: none

Dispersion: none


Stones Currently Available
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Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course