Nailhead spicule in yet another Natural emerald

by Chaman Golecha

"A STUDY OF NAIL-HEAD SPICULE INCLUSIONS IN NATURAL GEMSTONES" was our first mark in the index as a main article (earlier reports were in the short GNI section) of the Gems and Gemology (A quarterly of the Gemological Institute of America, USA). The article published is published in the current issue of the magazine (Vol.43, No.3, Fall 2007). One may read the abstract here.

Within few days of receiving the magazine, we encountered another Natural emerald with prominent true Nail Head spicules. Since, the article was already published and reporting this would be of no use, I decided to kick start my blog with this. What follows is a report on that emerald.

Figure: Nail Head spicule in Synthetic emerald, courtesy of GIA website.

We encountered this stone just 3 days back (10th Dec 07). The specimen weighing 3.71 cts, oval mixed cut was submitted for testing at the Gem Testing Laboratory, Jaipur, India. Initial analysis using standard gemmological techniques revealed properties (Refractive Index, Specific Gravity, Spectrum, Polariscope, Dichroscope reactions etc.) typical of emerald, Natural or Synthetic. Magnification is the most important feature in the separation of Natural and synthetic gemstones. This specimen was no exception.

Upon magnification, several parallel zones were observed with some elongated inclusions oriented perpendicular to the zones. Examination of these elongated feaures revealed NHS inclusions. This emerald had much clearer and well developed spicules than the one we had reported in the G&G.

Figure: Parallel zones and Nail head spicule inclusions. .

Some of the spicules had a very long cone along with clear phase. The base of the inclusions was made of cluster of crystal inclusions which somewhat appeared like flakes of some brown mineral. Due to lack of Raman, we could not determine the nature of the inclusions. All the inclusions were oriented along the optic axes which was determined by observation along crossed polars.

Though almost all the spicules had their cones pointing towards the same direction, a few of them were oriented at 180 degrees i.e. exactly opposite direction (still along the optic axes).

View along girdle revealed undulating features "chevron" effect as in the figure. Both the Nail head spicules and "chevron" growth features have traditionally been associated with hydrothermal synthetic stones. However this specimen proved to be an exception. The natural identity of this stone was made by the presence of zoning features, liquid films, crystal inclusions (mica?) etc. Further confirmation was obtained by FTIR which revealed features of Natural emerald.

Figure: Undulating pattern somewhat resembling chevron growth patterns.

Time and again, I have encountered features which have traditionally associated with Natural in Synthetic and vice-versa. Such specimens, time and again reminds one of careful and complete examination. Also one should never rely on a single feature to make an identification and such a stone could easily mislead a novice gemmologist.

P.S.: Click on the images to magnify them for a clearer view. Comments are welcome...!!

Acknowledgment: Mr. Gagan Choudhary and Mr. Mustaqueem Khan for all the co-operations and helpful discussions.

PHOTOMICROGRAPHS OF OTHER INCLUSIONS IN THE ABOVE SPECIMEN

More Nail head spicule inclusions:



Sheen observed along parallel zones: