For all practical purposes you have to assume that all emeralds are treated with oil, resin, polymer or a combination of more than one. Most Colombian emerald rough is now getting treated before it even reaches the cutter. This can be a problem for the faceter because they need to see the inclusions in order to orient the rough before cutting.
Are emerald treatments a problem for the consumer? Only if they are not disclosed or if someone is trying to sell you an untreated emerald at a premium. If someone tries to sell you an untreated emerald without proof from a major lab you should not buy it.
Remember emerald treatments have been going on for hundreds of years and are accepted worldwide in the trade as long as they are disclosed. A moderate level of treatment is the baseline for prices with heavily treated stones selling at a discount and lightly treated or untreated selling at a premium.
How can you tell the level of treatment without a lab report? The best way to do this is to count the number of surface reaching fissures on the crown of the emerald. Take out your loop and hold the emerald at an angle so that you can see a reflection off of the emerald’s table. Now look for little squiggly lines running across the table, as those are surface reaching fissures. The fissures are where the emerald treatment enters so roughly speaking the fewer fissures you have the less treatment has been introduced. Fissures are natural but while you do this you will also be looking for cracks. A crack that runs across any part of the emerald will be an automatic no buy. Look at a number of emeralds that are comparable and pick the one that has the fewest surface reaching fissures. An acceptable number of surface reaching fissures depends on the size of the gemstones and other factors but I shoot for about 0-3 in a 1-carat emerald. I’m more forgiving in larger emeralds but stricter in lighter colored emeralds.