Emerald is a green mineral. It is very closely connected to aquamarine (which is blue). They are both forms of a mineral called beryl. Beryl is called emerald only when it is transparent.

Beryl is made of columns of hexagonal crystals. The glassy green mineral is often found in large pieces, sometimes several metres long. They are formed by groups of six silica molecules stacked one above the other in sheets. Beryl is commonly found in granites, along with tourmaline.

Emerald gets its green colour because it contains the metal chromium, but this also makes it softer than other precious stones.