A ruby is a pink to blood-red coloured gemstone, a variety of the mineral corundum (aluminium oxide). The red colour is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. The ruby is considered one of the four precious stones, together with the sapphire, the emerald and the diamond. All natural rubies have imperfections in them, including colour impurities and inclusions of rutile needles known as "silk". Geologists use these needle inclusions found in natural rubies to distinguish them from synthetics, simulants, or substitutes. The Mogok Valley in Upper Myanmar (Burma) was for centuries the world's main source for rubies. That region has produced some of the finest rubies ever mined, but in recent years very few good rubies have been found there. The very best colour in Myanmar rubies is sometimes described as "pigeon's blood." In central Myanmar, the area of Mong Hsu began producing rubies during the 1990s and rapidly became the world's main ruby mining area. The most recently found ruby deposit in Myanmar is in Namya (Namyazeik) located in the northern state of Kachin. Rubies can be evaluated using the four Cs together with their size and geographic origin. In the evaluation of coloured gemstones, colour is the most important factor. Colour divides into three components, hue, saturation and tone. Hue refers to "colour" as we normally use the term. Transparent gemstones occur in the following primary hues: red, orange, yellow, green, blue, violet. The finest ruby is best described as being a vivid medium-dark toned red. Rubies have always been held in high esteem in Asian countries. They were used to ornament armour, scabbards, and harnesses of noblemen in India and China. Rubies were also laid beneath the foundation of buildings to secure good fortune to the structure.