[Gneiss] is a [metamorphic rock] that has parallel layers or bands. Some gneiss is made up of the same minerals (chiefly [feldspar] and [quartz]) as [granite], and it is then called granite gneiss. Several of the other kinds of gneiss are known as [mica] gneiss, [conglomerate] gneiss, gabbro gneiss, and hornblende gneiss. In order to be a gneiss, a metamorphic rock has to have bands or layers. These bands may be either straight or wavy and either wide or narrow. In most gneisses, you will find a layer made up of long or flat mineral grains next to a layer made up of the grains of an entirely different mineral. The bands may show color differences, too. For example, a pink layer made up of feldspar grains may be found next to a black layer made up of hornblende grains. The mineral grains interlock as they do in [igneous rocks], and they are generally large enough to be seen without a magnifying glass.
[Gneiss] from Blanco County, Texas, showing light and dark bands.
[Gneiss] can form from an [igneous] rock, such as [granite], or from a [sedimentary] rock, such as [sandstone]. Heat, [fluids], and pressures below the earth’s surface change these rocks into gneiss.
[Gneiss] that formed during [Precambrian] time is now seen at the surface in both west Texas and central Texas. In west Texas, it occurs principally in the Van Horn area of Culberson and Hudspeth counties. In central Texas, it is found in Blanco, Burnet, Gillespie, Llano, and Mason counties of the [Llano uplift] area.
One of the [Llano uplift] rocks is called the Valley Spring [Gneiss]. It generally has a light color (much of it is pinkish), and it is believed to have once been a [sandstone]. Another gneiss of this area, the Big Branch Gneiss, which has a medium to dark gray color, occurs in northern Gillespie and Blanco counties and is an altered [igneous] rock. Some of the Texas gneiss rocks are suitable for use as building stones.
Gold
[Gold] commonly occurs in nature as a single [element]—gold—but much native gold has a small amount of some other element, such as silver, copper, or iron, mixed with it.
Native [gold] is a shiny, yellow, metallic mineral that does not tarnish, and it leaves a shiny, golden-yellow [streak] when you rub it across a streak plate. If silver is present, the color and streak have a lighter shade. Pure gold is extremely heavy—its [specific gravity] is 19.3. Because it is malleable, this mineral will flatten into a thin sheet when hammered. It is ductile enough to be drawn out into wires. Gold is also soft—a pocket knife will scratch it easily. When it is to be used for ornaments and jewelry, gold is usually mixed with other metal, such as silver, copper, nickel, or palladium, to make it harder. The amount of gold that is present is then indicated by carats (or karats). Pure gold is 24 carats. If you have a gold ring that has 14 K stamped inside it, you know that it is made of a mixture of 14 parts gold and 10 parts of other metal.
[Gold] commonly occurs in nature as plates, scales, or grains. Some of the grains are large enough to be called nuggets. It also is found in a wire-like shape described as filiform, it occurs in a network, called reticulate, and it can have a branching and fern-like shape, described as dendritic. Gold is not often found as individual crystals.