ONYX
Onyx primarily refers to the parallel banded variety of the silicate mineral chalcedony that differ only in the form of the bands: agate has curved bands and Onyx has parallel bands. the color of its bands range from white to almost every color.
Commonly specimens of Onyx contain bands of black and/or white. Onyx as a descriptive term, has also been applied to parallel banded varieties of alabaster, marble, obsidian and opal, and misleadingly to materials with contorted banding, such as “cave Onyx” and “Mexican onyx”.
LIMESTONE
Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and molluscs.
Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).
About 10% of sedimentary rocks are limestone. The solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock.
Limestone has numerous uses: as a building material, an essential component of concrete (Portland cement), as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
TRAVERTINE
Travertine is a form of limestone deposited by mineral springs, especially hot springs.
Travertine often has a fibrous or concentricappearance and exists in white, tan, cream-colored, and even rusty varieties.
It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave.
In the latter, it can form stalactites, stalagmites, and other speleothems. It is frequently used in Italy and elsewhere as a building material.
MARBLE
Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism.
It is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. Under the conditions of metamorphism, the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals.
A related rock, dolomitic marble, is produced when dolostone is subjected to heat and pressure.
