The NCDIA, Natural Color Diamonds Association, created this colorful graphic showing the cause for each color. This doesn't apply exclusively to diamonds, as other gemstones acheive thie colors the same or similar ways
I spotted these paper diamonds over at MiniEco and will be giving them a try this weekend. How cool are they in a rainbow of colors? Pop over to Mini-eco for the printable template and complete step-by-step instructions.