Feldspar (also felspar, meaning spar of the field) is any of a group of crystalline minerals, made up mainly of aluminum silicates. They are glassy and moderately hard, and are found among igneous rocks. Spar is the name of various shiny materials that break off easily, in chips or flakes. Few of these varieties are used in ornaments, but the even milk-white tone of a good moonstone makes it effective in jewels. _Onyx_ Named, because of its pale color, from the Greek word for nail, onyx is a variety of agate. It consists of alternate layers of different colored stone, as can be seen around the edge; this makes it prized for carving, especially in cameos. _Opal_ The opal was represented in such variety in early times that the word _upala_ was the general Sanskrit term for a precious stone. The opal comprises a large group of vitreous, translucent silicas, possessing the property of refracting light and then reflecting it in a play of colors. Silica is a dioxide of silicon, chemically Si O₂, a hard glassy mineral that includes quartz and sand as well as opal. According as the compound includes iron, magnesium or other elements, the color of the stone varies. The best opals are the result of a flaw in their formation. Being hydrated silicas, they were at first a sort of semi-liquid, jellified substance; as this hardened, cracks and fissures were created by unevenness in the material and in the speed of the hardening. These tiny spaces trapped air or moisture, and it is this that produces the phenomenon of refraction and reflection of light and gives the colorings and variations known as opalescence. The play of light is at its best when the stone is cut cabochon, except for the fire opal, which is faceted. There are three chief varieties of opal. The common or white opal has a cloudy-white background, with pastel patches that often give it a veritable sunrise glow. The black opal has actually a very dark green background, in which there are deep pools of blue and green with patches of flame. Rare, and most magnificent, is the fire opal, which seems almost transparent, its body of smooth reddish orange shooting forth into flame. The opal is a delicate stone. It may be damaged by heat. It absorbs grease, and may thus become dull. The outstanding and valued feature of the stone is its opalescence. This creates a constantly changing, almost kaleidoscopic play of lights. It is this variability that gives point to the reference in Shakespeare’s _Twelfth Night_: “Now the melancholy god protect thee, and the tailor make thy garment of changeable taffeta, for thy mind is a very opal.” _Peridot_ The peridot, a yellowish-green variety of chrysolite, was popular in early England. It fell from favor but was reintroduced from France in the seventeenth century. It is a beautiful stone, often as large as 30 carats, and again growing in favor. _Quartz_