Commonly, sapphires are worn in jewellery. Sapphires may be found naturally, by searching through certain sediments (due to their resistance to being eroded compared to softer stones) or rock formations. The sapphire is one of the three gem-varieties of corundum, the other two being ruby, defined as corundum in a shade of red and padparadscha a pinkish orange variety. Although blue is their most well-known colour, sapphires may also be colourless and they are found in many colours including shades of grey and black. Colour in gemstones breaks down into three components, hue, saturation, and tone. Hue is most commonly understood as the "colour" of the gemstone. Saturation refers to the vividness or brightness of the hue, and tone is the lightness to darkness of the hue. Blue sapphire exists in various mixtures of its primary (blue) and secondary hues, various tonal levels (shades) and at various levels of saturation (vividness). Yellow and green sapphires are also commonly found. Pink sapphires deepen in colour as the quantity of chromium increases. The deeper the pink colour the higher their monetary value, as long as the colour is tending toward the red of rubies. Sapphires also occur in shades of orange and brown. Colourless sapphires are sometimes used as diamond substitutes in jewellery. Natural padparadscha (pinkish orange) sapphires often draw higher prices than many of even the finest blue sapphires. Recently, more sapphires of this colour have appeared on the market as a result of a new artificial treatment method that is called "lattice diffusion". A rare variety of natural sapphire, known as colour-change sapphire, exhibits different colours in different light. Colour change sapphires are blue in outdoor light and purple under incandescent indoor light, or green to grey-green in daylight and pink to reddish-violet in incandescent light. The sapphire is the traditional gift for a 65th Wedding anniversary.