emerald is a gemstone and a type of the mineral beryl colored green by trace amounts of chromium and sometimes vanadium. emerald in antiquity were mined in Egypt at locations on Mount Smaragdus since 1500 BCE.
yellow sapphire is a gem of the corundum family and is a twin of ruby and blue sapphire. it is available in yellow, golden and orange colors. the highest quality is known to be of lemon-yellow color. the color in yellow sapphire is of iron and titanium.
Alexandrite is known as "Emerald by day, ruby by night" because it has one of the most dramatic color changes among gemstones, such as green in sunlight and red in incandescent light. The very precious habit of alexandrite color change is caused by the presence of minor impurities in the element chromium. The most desirable Alexandrite colors are those that have a very clear color change.