GAO Yujie, Miro Ng,Charles Carmona, LIN Qingchun

 (Guild Gem Laboratories,Los Angeles, California 90013, USA)

 Abstract: Emeralds with highly saturated green color and pure clarity are strongly cherished and coveted by gem dealers and collectors. Those green beauties with the socalled“Gota de aceite”optical phenomenon are rarer. Rarity of such samples causes the scarcity of related research on this subject. This article reports on thegemological features of one 5.1ct emeraldwith distinct“gota de aceite”effect. Also, the possible cause of“gota de aceite”is described and illustrated in crystalline structural perspective. 

Key words: emerald;“gota de aceite”;color

In December 2016,Guild Gem Lab received a green gemstonering for testing. The submitted ring containsan octagonal shape faceted green stone, weighing 5.1ct indicated by the hallmark, with fancy cutdiamonds around as shown in Fig.1

Fig.1Agreen gemstonerin

 The standard gemological properties of the green stone were consistent with those of emerald. This stone exhibitsvivid green color under tungsten lamp and strong red colorwere seen through Chelsea color filter(CCF) as shown in Fig.2.The distinct contraction raised our doubts,as it is known that some synthetic emerald may exhibit strong red color under CCF.Aquick glance at this emerald under-microscope reinforced our hypothesis. Viewing perpendicular to the table,“zigzag-like” or“water ripple like” structures, which were similar to those growth structures in hydrothermal syntheticemerald, were casily observed, as shown in Fig.3.Similarbut not the same

 Fig.2 Vivid greencolor under tungsten lamp(leff) and strong red color throughCCF(right)

 Fig.3“Water ripple-like” structures

 In addition,there were little inclusion within this stone except for several small fractures near the girdle which were not seen by naked eyes, and high clarity caused more doubts in our mind. Suspiciousas we were and more precisely spcaking we were almost 90sure that this stone was synthetic, however,we still have to do more careful and meticulousobservation and testing for a 100% sure,following TheGuild Standard Testing Procedure.

1 Further observation and testing

In spite of the limitation of observing directions and conditions, for that this emerald was mounted in a ring, we still found something interesting when viewing nearthe girdle alongthe length of the stone, ormesmerizing to be precise. As shown in Fig. 4,“porphyritic” or“granulous” structureswere seen under high magnification 90X. Several spots were apparent with oily appearance.

Fig.4(a)“Porphyritic” or“granulous” structures

Fig.4(b) AreaA(lef) and areaB(right) showdistinct hexagonal outlines with saturated green color

 A further observation illustrated that the“spots” exhibit regular outlines and green color with different intensity. As shown in Fig.5~6, these two spot showed distinct hexagonal outlines, which is in good agreement with the crystal habit of beryl, which belongstohexagonal crystal system. Step-like growth structure was also observed, revealing the growing procedure of this sample.

Fig.5 Emerald crystalline structure viewing along c-axis     Fig.6 Emeraldcystalline structure combined by sixunits

Further testing of FTIR spectroscopy  and comparison Guild IR spectra database confirmed that this emerald was natural,asshown  in Fig.5.Chemical  compositions  of  major  and trace  elements  analyzed by  EDXRF with more than  330  spots applied were in good agreement with those ofColombian based on abundant data accumulated over three decades by Mr.Charles Carmona.

2 Analysis

The  combination  ofthe  whole  scenes,including  the  high clarity andoily appearance,were in very good agreement with the legendary  term  circulating  among  experienced  emerald  dealers and   seasoned   connoisseurs,called   “gota   de   aceite”originating from   Spanish   meaning“a    drop   ofoil”.According   to   Ronald  Ringsrud¹,this  expression  have  been  used  by  at  least  three generations of Colombian emerald dealers.And such optical effect may increase the value of an emerald.This phenomenon is also called“butterfly  wing  effect”(efecto  aleta  de  mariposa).So  far, such optical effect has only been reportedly found in Colombian emerald  from  variousdeposits,including  Muzo,La  Pita,Coscuez, and Penas Blancas mines.

However,this term isunfamiliar for the public mainly because this phenomenon is very rare and evens the stone do show such  effect only experienced gemologists and seasoned dealers can tell. Some emeralds with such qualities may be mistakingly identified as oiled oreven synthetic owing to the wavy structure and very high clarity.

To better elucidate this phenomena, the“gota de aceite” effect in Colombian emerald is like velvety effect in Kashmir sapphires. Additionally,although such scenes are only reportedly found in Colombian emeraldsat presentl²-5,itdoes not mean that emeraldsfrom  other localities  cannot  exhibit“gota  de  aceite”.As it is well known recently that Madagascar also produces sapphires withvelvety effect resembling those from Kashmir.

As material  science  develops,our  understanding  of crystal have been advancedgradually especially on different dimensional levels.Those crystals which arevisually clean by naked eye and  10X loupe may have several fractures when observed under 100X   microscope.Those  exhibit  no  inclusion  or  fractures  may  show clear cavities under greater magnification such as 1000X.The so-  called perfect crystals still possess certain crystal defect within the  crystalline structure,including the point defect,vacancy etc.

Thus,a solid foundation of knowledge of crystalline structure helps alot to understand the formation of“gota de aceite”effect  Emeralds,belonging  to  beryl  group(Be₃A₂Si₆Ojg)and  hexagonal crystal  system,exhibit  hexagonal  prism  with  the  cross  section perpendicular to C-axis being hexagonal,which agrees the outlines of  the“spots”mentioned above. Such  external appearances are  determined  by  the  internal  crystalline  arrangement,i.e.the crystallne structure.Fig.6 shows viewing along c-axis of emerald, and this viewing direction is identical to that of“gota de aceite”   effect.Six identical units composedof various  element atoms are clearly identified by three crossed auxiliary lines with cyan color  These units are consisted of Si and O as the main framework, consistent with the  fact  that  emerald  is  a  kind  of cyclosilicate combined by six Si-O tetrahedrons with one six-fold symmetry axis. Furthermore,as it is known that emerald is colored by Cr and/or Velements,when  Cr³+and  V³+substitute  theAl³+site.Connecting lines betweensix neighboring Al3+form a hexagonal ring,similar  to   the   outlines    of   the“spots”asshown   in   Fig.7.Hence,“gota de  aceite”effect  has  had  a  close  relation  with  the  crystalline structureof emerald and trace elements coloring the stone.