Emerald treatments: To enhance the clarity, color, brilliance and to make the stone more durable emeralds are generally treated with oil or resin. Almost all the emeralds have internal and surface reaching open fissures so filling them to make stone look more transparent, lusterous and colorful the aforesaid materials and sometimes other fillers are used. It should always be assumed that an emerald is treated unless it is proven.
Oiling is done on both rough and finished emeralds. The naturally occouring emerald rough has lots of internal cracks or fissures (internal tube like hollow spaces). For oiling simply an emerald is dipped in oil or oil is applied on surface and left for some time from few hours to a couple of days so that the air filled in the fissures is replaced by the oil as emerald has a property to soak the oil in it. The oil used for it can be colorless cedar wood oil, joban oil with color or any other mineral or vegetable oil.
Impact of oiling: After oiling the hollow inner cracks gets filled with oil vailing the shaggy appereance of the gemstone which was due to many fissures making the stone look whitish, weak and deteriorated. After oiling the stone looks more transparent as the fissures get intermingled with the rest of the crystals which were before seperated giving it uniform look which is more shiny and even the color is improved sometimes due to evenness bought up in the gem. The refraction of the light gets unified in the emerald cluster due to filling of the fissures thus giving it a better transparency and luster.
Minor and moderate (extent f1 - f2)- Here the emeralds have very little traces of oil in them which is not hiding any big cracks and only few fissures have oil which may not dry early. Even if the oil dries up in F1 - F2 graded gemstones the stone dont go a significant change in the appereance.
Significant oiling (extent F3) - Any such emerad with significant or heavy oiling will dry up early showing flaws in the stone with usually white patches which may be significant.
Note - Some gemologist gives oil grade just by the amount of fissures in emerald even if it does not have oil in it. IGI Jaipur certifiess most of minor oiled emeralds as moderate even if the emerald has no tendency to dry out and for same emerald GJEPC IIGJ (one of the best labs for color stones) for gives
As the oil dries up leaving the emerald in the old state it was before oiling is not a strong solution for enhancing the gemstone as oil inside do generally starts to dry up in couple of days to months depending on kind of cracks and fissures in the stone and the oiling has to be done time to time otherwise in some cases the stone can become vulnerable. A better alternative to oil is resin which also acts same like oil filling up the fissures but is more stable.
Note - In my many years of experience I have seen that in many cases the extent of oil given in certification does not match the quality of gem originally as the IGI or GIA can many times give a minor oiled emerald as moderate or significantly oiled stone and a significantly oiled specimen as moderate. Even the different laboratories have their own criterion for grading emerald on basis of treatment as some actually see the amount of oil in fissures present and some just by the amount of cracks or hollow space present in stone by which the potential to absorb oil is seen the grading is given.
In actual significantly oiled stones are those which dry up too much and whole appereance of stone changes when kept free from oil for certian period of time and big white patches or cracks starts to appear, but if a stone is not changing much keeping out of oil for a longer period of time it is surely minor oiled stone.
As we know that emeralds come with lot of cracks and fissures and lot are surface reaching so to cover them up traditionally many oils and natural rasins were used like cedar oil or canada balsam, but now epoxy or chemical resins are used in the place. Sometimes epoxy is used just to fill up a micro pit/pits on surface and the enhancement is very minor. Resin is a better way to treat emerald as it does not dries up quickely as the oil does but over a period of time the resin disintegrate, also it can be removed with some solvents if needed.
Resin and oil can combinedly be used as even resin treated emeralds are always oiled. So resin is a better solution for enhancing emeralds compared to oil in my openion.
Almost all emeralds are treated with resin or oil so if that is in minor amount then there is no change in emerald's price but if it has significant oiling the price drops less than half in respect of the quality seen. A minor or no oiled emerald if in 5 carats is of $1000/ct then same color/clarity emerald in significant oil or resin will be $600/ct.
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