On the origin of digit patterns in gem opal

Research areas:

• Surfaces planétaires (2011 - 2016)

Year:

2013

Authors:

• Benjamin Rondeau
• Jean-Pierre Gauthier
• Francesco Mazzero
• Emmanuel Fritsch
• Yves Bodeur
• Boris Chauviré

Journal:

GEMS & GEMOLOGY

Volume:

49

Number:

3

Pages:

138-146

Month:

FAL

ISSN:

0016-626X

BibTex:

@article{WOS:000331152800002, author = "Benjamin Rondeau and Jean-Pierre Gauthier and Francesco Mazzero and Emmanuel Fritsch and Yves Bodeur and Boris Chauvir{\'e}", abstract = "Some natural opals, mostly from Ethiopia, show a macroscopic finger-like structure called a digit pattern. This pattern consists of vertical columns that are more or less parallel, separated by a homogeneous matrix of different color, transparency, or play-of-color. This study proposes that digits develop through: (1) the deposition of a homogeneous opal layer and subsequent polygonization in the form of vertical columns; (2) preferential alteration of this layer at the vertical grain and sub-grain boundaries, creating the digit shape; (3) precipitation of a new silica gel in the space between the digits; and (4) the drying and solidification of the opal. Although polygonization in the form of vertical columns is a growth process typical of synthetic opal, the post-growth alteration of these columns into digits and the deposition of matrix are observed only in natural opal.", doi = "10.5741/GEMS.49.3.138", extra = "PICL", issn = "0016-626X", journal = "GEMS {\&}amp; GEMOLOGY", month = "FAL", number = "3", pages = "138-146", title = "{O}n the origin of digit patterns in gem opal", volume = "49", year = "2013", }

Abstract:

Some natural opals, mostly from Ethiopia, show a macroscopic finger-like structure called a digit pattern. This pattern consists of vertical columns that are more or less parallel, separated by a homogeneous matrix of different color, transparency, or play-of-color. This study proposes that digits develop through: (1) the deposition of a homogeneous opal layer and subsequent polygonization in the form of vertical columns; (2) preferential alteration of this layer at the vertical grain and sub-grain boundaries, creating the digit shape; (3) precipitation of a new silica gel in the space between the digits; and (4) the drying and solidification of the opal. Although polygonization in the form of vertical columns is a growth process typical of synthetic opal, the post-growth alteration of these columns into digits and the deposition of matrix are observed only in natural opal.