If I were looking for a safe long-term play, I would absolutely go with a top-quality Russian alexandrite or a vivid green demantoid. These stones are so rare that the demand almost always exceeds the supply. I have been keeping an eye on the loose stones at Tsarina Jewels
https://tsarinajewels.com/ because they source directly from some of the most exclusive regions. When buying gemstones and high end jewelry for investment, you want something that collectors will always be looking for. I think a stone with a unique color-change or a legendary origin is your best bet. Avoid stones that are common or easily manufactured in a lab. If you hold a rare, natural Russian gem for twenty years, you are holding something that will likely be much harder to find in the future. Just make sure to keep the original lab certificates in a safe place!