There is not a unity of opinion concerning the cause of this difference. “Their colors,” says Cronstedt, “are commonly supposed to depend upon metallic vapors; but may they not more justly be supposed to arise from a phlogiston united with a metallic or some other earth? because we find that metallic earths which are perfectly well calcined give no color to any glass; and that the manganese, on the other hand, gives more color than can be ascribed to the small quantity of metal which is to be extracted from it.” M. Magellan is of opinion, that their color is owing chiefly to the mixture of iron which enters their composition; but approves the sentiment of Cronstedt, that phlogiston has a share in their production, it being well known that the calces of iron when dephlogisticated, produce the red and yellow colors of marble, and when phlogisticated to a certain degree produce the blue or green colors. With regard to the texture of gems, M. Magellan observes, that all of them are foliated or laminated, and of various degrees of hardness. Whenever the edges of these laminæ are sensible to the eye, they have a fibrous appearance, and reflect various shades of color, which change successively according to their angular position to the eye. These are called by the French _chatorantes_; and what is a blemish in their transparency, often enhances their value on account of their scarcity. But when the substance of a gem is composed of a broken texture, consisting of various sets of laminæ differently inclined to each other, it emits at the same time various irradiations of different colors, which succeed one another according to their angle of position. This kind of gems has obtained the name of _opals_, which are valued in proportion to the brilliancy, beauty, and variety of their colors. Their crystallization, no doubt, depends on the same cause which produces that of salts, earths, and metals: but as to the particular configuration of each species of gems, we can hardly depend upon any individual form as a criterion to ascertain each kind; and when we have attended with the utmost care to all that has been written on the subject, we are at last obliged to appeal to chemical analysis, because it very often assumes various forms.[113] * * * * * The rich treasures of the earth are within it, observes a worthy author, so that they cannot be discovered and brought to the surface without the labor of man; yet they are not placed so deep, as to render his exertion ineffectual. Thus nothing but what is comparatively worthless is to be found by the indolent on the surface of life. Every thing valuable must be obtained by diligent research and sedulous effort. All wisdom, science, art and experience, are hidden at a proper depth for the exercise of intellect, and they who bend their attention to any of these objects shall not be disappointed in their pursuit.