Roger Weller, geology instructor
Quartz Family Gemstones-by Audrey LaClair
Chert is a form of microcrystalline quartz. It’s colors are typically white, but can range from many colors. The color can sometimes lead to problems as far as naming it a material chert rather than a jasper, although some consider jasper to be a color of chert.
“Chert is a rock type composed mostly of the mineral chalcedony, cryptocrystalline silica, or quartz in crystals of submicroscopic size. It can form in parts of the deep sea where the tiny shells of siliceous organisms are concentrated, or where underground fluids replace sediments with silica”
Chert is also found in nodules in limestone, which is the most common site. It’s color there is a light to a dark gray. Chert may have formed from remains of very old sea sponges or other sea animals that have been fossilized. Chert is actually the proper name for arrowheads, some spears and knives, as well as other stone tools. It is found to be rich in silicon dioxide. Flint, which is the actual arrowhead, comes from a very dark form of chert.
Chert pebbles that are striped can be found along the California coast. The pebbles are layered with impurities, which could be volcanic ash, clay minerals, or silty sediment, but it is difficult to tell which one without a microscope.