Quartz
2 of the 7 encyclopedias on this shelf carry an entry for Quartz. Both are reproduced below, so you can see where they agree and where they differ.
The Encyclopedia of Founding (1892)
The purest condition of silicon is that of quartz, in which it forms hexagonal crystals terminated by six-sided summits. It is a very abundant and widely diffused mineral. Quartz rock is mainly composed of it, and it is in this substance that gold is more frequently found than in any other. Quartz is the principal constituent of nearly all granites, as well as the numerous sandstones, limestones, trap-rocks, etc. Sands of both desert and sea-shore, and the common flint, are chiefly composed of it; and many stones, as the agate, amethyst, carnelian, chalcedony, jasper, rock-crystal, etc., are simply varieties of quartz. It is thought that a great deal of the silica which exists in nature has been originally deposited in the soluble condition. The structure of the chalcedony, etc., proves that they were formed by a solution of silica having penetrated into a cavity in the surrounding rock and there crystallized. See throughout this work for a description of the substances mentioned.
Aiton's Encyclopedia (1910)
a mineral compound of silicon and oxygen. It is one of the most abundant rock materials, constituting large masses, as well as the sands of the desert and the seashore. Crushed it forms a beautiful sand. It enters into the make-up of granite, gneiss, quartzite, and sandstone. It is hard, scratching glass easily, and cannot be melted at ordinary temperatures or dissolved by ordinary acids. For this reason, glass, which is chiefly quartz, is valuable in a laboratory and in the household. Many varieties of pure quartz are colored, as rose quartz, the opal, the amethyst, the smoky quartz, chalcedony, sardonyx, agate, carnelian, onyx, and agatized wood. Pulverized quartz is frequently used in preference to sand, being clearer and purer, for making glass and porcelain. It is also used for making sandpaper, as the crushed particles have a fresher, sharper fracture than worn sand. The mineral known as pebble, used for spectacles, is a transparent quartz. See Sand ; Silicon