...not all gems are built like a diamond, and so they should not always be cut like one. And if you think so too, you're not alone. History agrees. Take a look at the rings of royalty throughout history, and you'll see masterfully polished, rounded colored gems. Saturated sapphires and aquamarines set in magnificent golden adornments were the norm for millennia. The mineral structures of a beryl (emerald, aquamarine, morganite) are completely different from those of a corundum (sapphire and ruby). Because of that, light will behave differently once it enters the gem. The cutter should take this into account. What has happened over time is that cuts from the diamond chart have been applied to the crystal structures of other gems and it almost seems like putting antlers on a bear.