Muscovite. See [Mica].
Native Silver. See [Silver Minerals].
Obsidian and [Vitrophyre]
[Obsidian] is a dark, glassy-looking [igneous] rock. Most obsidian contains the same chemical [elements] as [granite] and [rhyolite], since all three of these rocks can form from the same type of molten rock material. Obsidian, however, has no separate minerals, because its chemical elements are not combined in an orderly way. It is a natural glass.
Because it is a glass, we know that [obsidian] forms very quickly. One way for it to form is from the sudden cooling of hot, molten [lava] that flows out of volcanoes. If the lava cools and hardens before the separate minerals can crystallize, it becomes a natural glass, such as obsidian.
This rock is smooth and shiny. Most of it is black, but some can be dark green or dark brown. [Obsidian] allows light to pass through it, and it breaks with a curved, [conchoidal] [fracture]. The broken edges are very sharp.
Another glassy [igneous] rock that forms from fast-cooling [lava] is [vitrophyre]. It looks like [obsidian] except that it has crystals or [crystalline] mineral grains (which may be light colored) scattered through the dark glassy material.
[Obsidian] was used by the Indians to make arrowheads.
[Obsidian and vitrophyre] are found in the Big Bend area of Brewster and Presidio counties in west Texas. They occur with other [igneous rocks] that formed there during [Tertiary] time.