For a more complete rock determination chart, you can consult a textbook, such as Rocks and Rock Minerals, by L. V. Pirsson and A. Knopf.
Rock Identification Charts
| Chart No. | Rock | Remarks | ||
|---|---|---|---|---|
| 1. | GLASSY appearance (rock is dark, smooth, and shiny) | |||
| [Obsidian] | Entire rock is glassy | |||
| [Vitrophyre] | [Crystalline] grains are scattered through the dark glassy mass | |||
| 2. | COMPACT, DULL, OR STONY appearance (individual grains too small to be recognized) | |||
| A. | Can be scratched by a fingernail | |||
| [Chalk] | Dilute hydrochloric acid fizzes on it | |||
| [Clay] | Earthy odor when breathed on | |||
| [Diatomite] | Crumbly | |||
| [Rock gypsum] | Made up of the mineral [gypsum] | |||
| [Soapstone] | Soapy or greasy feel | |||
| B. | Cannot be scratched by a fingernail but can be scratched by ordinary pocket knife | |||
| [Dolomite] | Dilute hydrochloric acid may fizz slightly on it; will fizz if rock is powdered | |||
| [Limestone] | Dilute hydrochloric acid fizzes on it | |||
| [Serpentine] rock | Commonly some shade of green | |||
| [Shale] | Breaks in flat, thin flakes; earthy odor | |||
| C. | Cannot be scratched by an ordinary pocket knife | |||
| [Basalt] | Dark colored and heavy | |||
| [Chert] | Hard, smooth, and porcelain-like | |||
| [Rhyolite] | Light to dark colored; may show streaks or flow structure | |||
| 3. | [GRANULAR] appearance (at least some of the individual grains are large enough to be seen without a magnifying glass) | |||
| A. | Can be scratched by an ordinary pocket knife | |||
| [Limestone] | Dilute hydrochloric acid will fizz on it | |||
| [Marble] | Dilute hydrochloric acid will fizz on [calcite] [marble], and it may fizz slightly on [dolomite] marble | |||
| [Rock gypsum] | Made up of the mineral gypsum | |||
| [Rock salt] | Has a salty taste; made up of the mineral [halite] | |||
| B. | Generally cannot be scratched by an ordinary pocket knife (some [schist] is softer) | |||
| 1. | Grains are of about equal size (equigranular) | |||
| [Granite] | [Quartz] and [feldspar] grains interlock | |||
| [Pegmatite] | Large interlocking grains; commonly feldspar, quartz, [mica] | |||
| [Quartzite] | Rock breaks across the quartz grains | |||
| [Sandstone] | Rock breaks through the cement but around the [sand] grains | |||
| 2. | Easily seen grains are scattered through a mass of finer grains | |||
| [Basalt] | Dark colored, heavy rock | |||
| [Llanite] | Rusty pink feldspar and blue quartz grains embedded in a brownish rock mass | |||
| [Rhyolite] porphyry | Light to dark colored rock; may show streaks or flow structure | |||
| 3. | Grains are arranged in layers | |||
| [Gneiss] | Interlocking grains are in straight or wavy bands | |||
| [Schist] | Splits in thin layers; some schist is soft enough to be scratched by a knife | |||
| 4. | FRAGMENTAL appearance (rocks are made up of fragments that are either loose or cemented together) | |||
| [Breccia] | Angular, gravel-size fragments that are cemented together | |||
| [Conglomerate] | Rounded, gravel-size fragments that are cemented together | |||
| [Coquina] | Shells and shell fragments that are cemented together | |||
| [Gravel] | Loose fragments | |||
| [Pulverulent limestone] | Loose, powdery fragments; dilute hydrochloric acid fizzes on them | |||
| Sand | Loose fragments no larger than a pinhead | |||
| [Sandstone] | Sand fragments that are cemented together | |||
| [Volcanic ash] | Loose, fine, gritty particles | |||
[Physiographic outline map] of Texas.
DESCRIPTIONS OF SOME TEXAS ROCKS AND MINERALS
The pages that follow contain descriptions of Texas rocks and minerals. The descriptions are given in alphabetical order, except that related varieties are described together. For example, [agate], [amethyst], [chert], [flint], [jasper], [onyx], and [chalcedony] are discussed under [quartz], because they are varieties of quartz. The descriptions include the properties of the rock or mineral that will help you identify it and also include information on where the rock or mineral can be found in Texas, some of its uses, and how it may have formed. The chart on [page 99] lists chemical composition, [specific gravity], and hardness of various Texas minerals.
Agate. See [Quartz].
Agatized Wood. See [Quartz].
Alabaster. See [Gypsum].