When shopping for gold, you’ll always see it spoken of in terms of karats: 24k, 18k, 12k, and so on. But what exactly is a karat? What does it measure, and how does it compare to the confusingly similar “carat”?

In my last blog, I talked a little about tsavorite, one of the rarest gemstones on Earth, mined in only a small corner of Kenya and Tanzania. Tanzanite is another rare beauty from East Africa, perhaps even rarer than tsavorite. If you’re looking for tanzanite, you’ll find it only in an eight-square-kilometer region that lies in the shadow of Mount Kilimanjaro.

Few gems match the pure, rich green of emerald, but the tsavorite variety of garnet comes close. When we think of garnet, we probably think of a gemstone ranging in color from caramel brown to pomegranate red, but garnet appears in just about every color of the rainbow. Tsavorite, which ranges in color from olive to evergreen (the latter being the most valuable), is one of the rarest shades of all—and one of the rarest gemstones in the world.

As the snow melts and the trees sprout leaves in May, we start to see the soothing, rich green of emerald all around us. If you feel the same sense of relaxation looking at an emerald that you do sitting in a green field in the springtime, it’s not just your imagination. Emeralds are said to relieve exhaustion and bring a sense of tranquility to the wearer, and like many of the properties often attributed to gemstones, this idea has some scientific basis.