Jasper is the common name for slightly translucent to opaque silica based minerals. The same mineral is commonly referred to as agate when it is more transparent.

Agates and jaspers are very similar to quartz minerals such as amethyst and citrine. The primary difference is the way the mineral has crystalized. In the case of quartz, larger crystals are formed. These larger crystal structures can be seen in uncut quartz stones. With jasper, the crystals are so small that they cannot be see with the naked eye. This causes uncut jasper stones to have an irregular, bloby shape, instead of the clearly defined quartz crystal shapes. Light, reflecting off many tiny crystals, gives the polished stones a softer, more pastel colored look.

The different colors of jasper come from other minerals within the stone. Many of the greens are from mica, the beige and brown colors are often iron oxide. Because of the way jasper is formed, other minerals are incorporated easily within the stone. This results in jaspers and agates in nearly any color imaginable. From jet black (black onyx), to green, to red (red jasper and carnelian). In the case of the amethyst and jasper bracelet above, many different colors of jasper are combined with amethyst.

Many fossils are made of jasper. In this case, silica replaces the original living item, and becomes a jasper stone which has the shape of the original item.

There are many common names for different types of jasper, such as bloodstone, and picture jasper.