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.