[Azurite] and [malachite] occur as individual crystals, but you are more likely to find them as crusts on rocks and on other minerals. Malachite is also found in rounded fibrous masses that resemble bunches of grapes (described then as botryoidal).
Both [azurite] and [malachite] are formed in the same way. Underground waters seep through rocks that contain deposits of [copper minerals] (such as [chalcocite] and [chalcopyrite]) and cause chemical reactions which change these minerals into malachite and azurite.
[Malachite] is more plentiful than [azurite], but both minerals can be found together. You can expect to find at least one of them at the same localities where [chalcocite], [chalcopyrite], and other [copper minerals] occur.
Coquina. See [Limestone].
Diatomite. See [Opal].
Dolomite
[Dolomite] is the name given both to a rock and to a mineral. The mineral is a calcium-magnesium carbonate and has a glassy or a pearly luster. It is any of a number of colors, such as white, pink, brown, or gray, or it can be colorless. Dolomite leaves a white [streak] on a streak plate and is [transparent] to [translucent]. It is not particularly hard and can be scratched with a pocket knife, although not with a copper penny. Dolomite cleaves perfectly in three directions, and some of the [cleavage fragments] are rhombohedrons. However, the [cleavages] of the individual mineral grains in specimens of fine-grained [massive] dolomite are not readily distinguishable.
[Dolomite] rock from the vicinity of Fairland, Burnet County, Texas.
Most Texas [dolomite] occurs as coarse-, medium-, and fine-grained [crystalline] masses as the chief mineral in dolomite rock and in dolomitic [marble]. It is also found as 6-sided crystals that are rhomb-shaped; when the faces are curved, they have a saddle-like appearance.