In Texas (USA) you can still notice metates lying on the desert sand. Those metates are flat grinding stones used by American Indians to grind corn. Metates together with handheld stones called manos are still used for this purpose.
Metates and manos are survivors of the Stone Age when people used rocks to make a wide variety of specialized tools and weapons. Chert, especially the dark variety known as flint, was among their favorite raw materials. Nodules of flint and chert can be easily shattered with a stone hammer or fractured by applying pressure with a stone or a deer antler. Stone age artisans used these methods to skillfully fabricate razor-sharp knife blades, scrapers and arrow and spear points.
     In Texas, flint and chert can be found lying on the surface or in seams in limestone. Seams of flint several inches thick can be seen in several of the road cuts along Interstate 10 west of Junction. In places, the chert emerges at the surface, where Stone Age people could collect it. Quarries could be dug to collect even more. During Stone Age times, the trading of both raw chert and the tools and weapons made from it was a major form of commerce. Archaeologists have traced ancient trade routes by identifying the origin of the chert and flint used to make stone tools and weapons.
     Over the centuries, stone-tipped spears and arrows were replaced by those having metal tips. But flint remained in use. Steel struck by flint produced a spray of tiny molten balls of metal, and such sparks were used to ignite fires.After the invention of firearms, flint was used to ignite the black powder that propelled bullets through gun barrels. When the Spanish, French and English did battle with American Indians who were still living in the Stone Age, both sides employed weapons equipped with flint. One side used ancient flint-tipped arrows. The other used flint to fire its guns.
     The geologic origin of chert and flint is much more uncertain than the vital role they played in human history. Some think that chert and flint were formed much like petrified wood in a process in which quartz replaced limestone under the floor of an ancient sea. Others believe that chert was formed when tiny quartz grains were precipitated directly from ocean water. A more recent theory holds that chert and flint were formed by wind-driven grains of quartz that fell into ancient oceans. According to C. Blaine Cecil of the U.S. Geological Survey, dust from the Sahara Desert is mainly quartz, and this might explain layers of chert found in England and the southeastern United States.

Source: My San Antonio News (23 February 2009)