I had a customer recently ask if his gemstone would always retain it’s value. We certainly hope so. But to be honest, it’s hard to say. Like most things, gemstone prices are driven by supply and demand, quality and rarity etc.
In the lab-grown market, technology is a key factor in determining price. Right now, large lab-grown pink and blue diamonds are rare and pricey because the cost to grow them is high. However, as technology improves, they may be more readily available in the future, which may drive the price down. Brand names, such as Chatham, add value because the stones are backed by quality guarantees etc.
In the natural market, a gemstone that is rare today may not be as rare in the future if a large deposit of that specific rare gem is discovered.
No one can say with absolute certainty if a stone will maintain its value in the future. I believe diamonds and gemstones should be purchased as investments in the marriage or relationship rather than the stone. 
Author: Michelle M. Rahm is a GIA-trained Graduate Gemologist. She has been selling gemstones and jewelry online since 1997. Visit her websites JewelryImpressions.com and OurCustomWeddingRings.com
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