1. Opal: Fragile opal is popular for costume rings. Its opalescence is caused by the diffraction of light off of tiny silica spheres inside the stone. Opals with more red tones are usually the most valuable.
2. Amethyst: The purple variety of the mineral quartz. Its color ranges from lilac to deep purple.
3. Pearl: Formed in mollusks, pearls begin as an irritant (such as sand) that becomes coated with an iridescent substance. These delicate gems can vary from white to pink to brown to black.
4. Turquoise: This opaque gem ranges in color from sky-blue to green. A soft material; not suggested for rings worn daily.
5. Topaz: Available in a variety of colors and prices, from the most valuable yellow or pink to the more affordable blue topaz.
6. Diamond: The hardest mineral and most popular for engagement rings. Comes in hues like pink, yellow, orange, and green, though rare. The fewer the flaws, the rarer the stone.
7. Peridot: This gem can be yellow-green to deep olive. Not a hard stone; can scratch easily.
8. Ruby: Red variety of the hard mineral corundum. Second only to diamonds in hardness; one of the best stones for a ring.
9. Emerald: A green form of the mineral beryl. A sturdy stone. Rarely perfect; oiled to enhance color and obscure flaws.
10. Sapphire: Blue form of the mineral corundum. The most prized color is a clear, deep blue. As strong as a ruby.
11. Aquamarine: Shades can vary, but most valuable are sky blue and dark blue. A hard, clear, moderately priced gem.
12. Garnet: One of most diverse gem families. Found in greens, reds, yellows, and oranges. Deep red is most common.