Pearls being considered lesser gems could be based off the fact that pearls are not "real" gemstones, but are instead organic, being formed by mollusks. As organic gemstones, pearls belong to the same group as ammolite, coral, ivory, jet and amber. These are all formed as a result of biological processes. Pearl is one of three birthstones for the month of June as adopted by the American National Association of Jewelers in 1912. It is also the birthstone for the Sun Signs of Gemini and Cancer. Pearls are also given on the third and thirteenth anniversaries. Pearl is the national gemstone of the Philippines. The name "pearl" is said to have originated from the Middle English word "perle", which in turn came from the Latin word "perna", meaning "leg", thought to be due to the ham-leg shape of the bivalve mollusc. Pearls vary in color from white to those with a hint of color, often pink, to brown or black. Each coloration will depend on the type of mollusk and the water where the mollusk lived. Pearls come in white, off-white, cream, golden, peach, pink, black, silver, lavender, and blue. Some pearls exhibit iridescence, which is known as orient. Orient can be predominantly one color such as green or pink, or consist of a rainbow of colors in weak, moderate, or strong appearances. The colors of pearls also have sometimes been associated with certain qualities: black or gold with wealth, blue with love, pink with success. Pearls have been harvested and cultivated primarily for use in jewelry, but in the past were also used to adorn clothing. They have also been crushed and used in cosmetics, medicines and paint formulations. Pearls are thought to give wisdom through experience, to quicken the laws of karma and to cement engagements and love relationships. They are thought to keep children safe. Pearls are quite "soft" and range between 2.5 and 4.5 on the Mohs scale of hardness. Because the nacre is organic, pearls are very sensitive to extreme heat, acids, dryness, and humidity. Although pearls are relatively soft, they are extremely compact, which makes them durable and resistant to being crushed.