Chart that shows the chemical composition of various gemstones. The chart lists the following gemstones: alexandrite, amethyst, aquamarine, diamond, emerald, garnet, opal, pearl, peridot, ruby, sapphire, spinel, topaz, tourmaline, turquoise, and zircon.

For each gemstone, the chart lists its chemical formula, hardness on the Mohs scale, and the cause of its color.

• Chemical formula: The chemical formula of a gemstone is a way of representing the elements that make it up and the proportions in which they are combined. For example, the chemical formula for diamond is C, which means that a diamond is made up entirely of carbon atoms.

• Mohs hardness: The Mohs scale is a system for ranking the hardness of minerals from 1 (softest) to 10 (hardest). A mineral's hardness is determined by how easily it can be scratched by another mineral. For example, a mineral with a Mohs hardness of 5 can be scratched by a mineral with a Mohs hardness of 6 or higher.

• Cause of color: The color of a gemstone is caused by the way it absorbs light. When white light shines on a gemstone, some wavelengths of light are absorbed by the gemstone, and other wavelengths are reflected or transmitted. The wavelengths of light that are reflected or transmitted give the gemstone its color. For example, an emerald appears green because it absorbs red and yellow light and reflects green light.

Here is a more detailed explanation of the information in the chart for a few of the gemstones:

• Alexandrite: Alexandrite is a variety of the mineral chrysoberyl. Its chemical formula is Al2BeO4. Alexandrite is known for its color change, which is caused by the presence of chromium. In daylight, alexandrite appears green because it absorbs red light. In incandescent light, alexandrite appears red because it absorbs green and blue light.

• Amethyst: Amethyst is a variety of quartz. Its chemical formula is SiO2. The purple color of amethyst is caused by the presence of trace amounts of iron and manganese.

• Aquamarine: Aquamarine is a variety of the mineral beryl. Its chemical formula is Be3Al2(SiO3)6. The blue color of aquamarine is caused by the presence of iron.