The boiling waters of the Iceland geyser are projected into the air at a considerable height, and are heavily charged with silica. As the waters fall upon the earth, large piles of earthy and stony material are formed in process of time. When these silicious masses are broken open, translucent and transparent portions of silica are found, exhibiting the colored reflections of the noble opal as long as they remain hydrated, or, in other words, as long as they retain a certain quantity of water in their composition. This observation has led M. Descloizeaux to the belief that opals found in volcanic rocks or igneous rocks have had their origin in phenomena analogous to those of the Iceland geysers. The matrix of the opal is a varied one. The gem is not only found in porcelain earth, but it occurs in fissures and seams, in what appear to be old igneous rocks. It has also been deposited in recent periods, as in the limestones of the argillaceous beds, and even in the formations of the silicious waters of the hot springs of the present time. The decomposed cement of the old Roman ruins around the hot springs of Polombieres, uniting with certain chemical properties of the waters, has changed into opal and hyalite. Trees within historic times have been transformed into opal or semi-opal; and on the island of Unja one may see blocks and trunks of trees (some even showing the marks of the hatchet) converted into opal. Silicified trees forty or fifty feet in length, may be seen stretched from Cairo to Suez. In many other parts of the world trees and plants have been transformed by the mysterious processes of nature into a silicious substance possessing the characters of opal; but none of these vegetable metamorphoses exhibit the rainbow hues to any marked degree. Quartz, when flawed in the interior, sometimes exhibits a remarkable iridescence, and may imitate the opal, especially if viewed at a distance. Such specimens of iridized quartz are called “iris,” and they may be artificially produced by a sudden blow upon the stone, or by heating it and suddenly dropping it into cold water. The superb iris ornaments worn by the Empress Josephine were of remarkable brilliancy and play of colors. In ancient and mediæval times, iridescent quartz was held in great esteem; and fine specimens mounted in antique jewelry are preserved at the present day. It is described in the “Lapidarium” of Marbodeus as follows:-- “By the Red Sea the swarthy Arabs glean The iris, splendent with the crystal’s sheen; Its form six-sided, full of heaven’s own light, Has justly gained the name of rainbow bright.”