They can be if you buy at a good price and buy the right stone.
Buying a diamond as an investment must start with the right diamond a traditional cut like a round brilliant in my opinion would be best. Certification from a lab is key GIA is the best in our experience. Finding the right color,and clarity is very important can’t stress that enough. Colored diamonds are rare and would make a great diamond investment fancy colored diamonds the rarest of these would be purple, pink, and blue. Only those colored from nature not color manipulated in a lab.
If you’re looking to invest in diamonds remember do your home work don’t just rush in head long understand what you’re buying.