Gemstones are often referred to as, being natural, genuine, synthetic, simulated, and treated or a number of those terms.
The term natural gemstone refers to gemstones which come from nature with out interference or changes by the jeweler other then for cutting and polishing.
Genuine gemstones are natural or genuine gemstones which could have been treated to enhance their appearance, however they are real gemstones.
Remember the quality of stones is most important as a poor quality stone is not worth the same value as a stone of the same size of high quality.
Synthetic gemstones share the appearance of a natural stone, however are created in a lab allowing for lower cost, and available in many types of gemstones that are on the market.