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.

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May, 2006 Chrysoprase

Chrysoprase is chalcedony (microcrystalline quartz) whose rich green color is derived from the presence of minute particles of a nickel containing mineral, willemseite. The derivation of its beautiful color is, then, analogous to chrysocolla chalcedony's coloration by particles of the copper mineral chrysocolla. Chrysoprase, with nickel as a chromophore, is notable among green stones as the majority are colored by iron (like peridot), or chromium (like emerald and chrome tourmaline), or vanadium (like Tsavorite garnet).

Geologically it forms as a precipitate from solutions containing silica and nickel compounds, generally derived from the weathering of serpentines. These solutions crystallize in fissures, cracks and cavities in various types of rocks. Historically deposits have been found in Eastern Europe, the US, Russia and Brazil, but by far the lion's share of today's World chrysoprase production comes from Australia.

The name, derived from Greek for "golden + leek" belies the green color, but may have reflected either yellowish local deposits, or as so often happens in gemological antiquity, the transfer of names from one gem material to another as time passed. Colors range from near emerald, to apple, to leek green, with or without matrix, and diaphaniety ranges from nearly transparent to opaque.

[Chrysoprase extremes: from nearly transparent in these carved leaves to opaque with black matrix in the cabochons]

At one time the duller, leek green stones were called prase, and the more vivid apple greens ones chrysoprase, but this distinction is given little attention today.

[Roughs of apple and leek green material which may sometimes be distinguished as chrysoprase and prase]

Most commonly we encounter this gem as translucent cabochons or carvings in its apple green color.

[Cabochons and carvings of fine quality chrysoprase]

The appearance of the best colored pieces is similar to that of fine jade, and explains the high regard this stone inspires in the Orient. In fact, unscrupulous sellers of both contemporary and vintage jewelry have been known to sell chrysoprase as the more valuable jadeite. Looking at the two rings below, it is not immediately obvious which is chrysoprase and which is jadeite.

[Chrysoprase ring, jadeite ring]

A rare, related chalcedony in darker, less saturated shades of green, that is colored by chromium occurs in Zimbabwe and is called Mtorolite, but sometimes is sold as "African chrysoprase".

[Mtorolite chalcedony from Zimbabwe]

Like other forms of chalcedony, chrysoprase makes a good, tough, gem for all jewelry applications. The only concern is that some specimens may lose a bit of their color if they are exposured to prolonged high heat or intense light. Chrysoprase, itself, is not known to be treated or enhanced; however dyed green agate, and green glass have been commonly used as simulants.

Value

With the exception of the finest grades of chrysocolla chalcedony, chrysoprase is the most valuable variety among the chaledonies. The most desirable gems in this material feature an even, highly saturated color with substantial translucency and no visible inclusions. Custom cut specimens are more valuable than calibrated cuts and larger pieces of high quality are rare.


Gemological Data:

Makeup: Silicon Dioxide

Luster: Vitreous to waxy

Hardness: 7

Crystal structure: Trigonal

Fracture: conchoidal

Cleavage: none

Density: 2.61

RI: 1.53-1.54

Birefringence: .004

Pleiochroism: none

Stones Currently Available:

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Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment