Travertine (used in construction, not a 'specimen' in situ)

Hardness: 3-4 (Mohs); Color: White, tan, cream, yellow, brown; Luster: Dull to vitreous; Crystal structure: Microcrystalline to cryptocrystalline, porous; Cleavage: None apparent; Specific Gravity: 2.3-2.7

Formed by the precipitation of carbonate minerals (primarily calcite) from geothermal hot springs or cold springs. Water carrying dissolved calcium carbonate emerges from the ground, and as carbon dioxide escapes, the calcium carbonate precipitates, often trapping detritus and organic matter. This process can be relatively rapid on a geological scale. The travertine used in the Colosseum, for instance, comes from quarries near Tivoli, Italy, which have been active since Roman times, and the deposits themselves date back tens of thousands to hundreds of thousands of years.

Extensively used as a building material, especially in ancient Roman architecture (e.g., Colosseum, Trevi Fountain). Also used for flooring, wall cladding, countertops, and decorative purposes due to its attractive appearance and relative ease of shaping. Less commonly used in jewelry, but polished pieces can be found.

The Colosseum in Rome (seen in the image) is one of the most iconic structures built almost entirely from travertine, showcasing its durability and aesthetic appeal over millennia. Its porous nature can lead to interesting patterns and variations in color. Travertine is a terrestrial sedimentary rock, meaning it forms on land, distinguishing it from marine limestones.

In the field (or in a building like the Colosseum), travertine is identified by its typically light, earthy colors (creams, tans), notable porosity (often with visible small holes and cavities), and banded or layered appearance. It often feels somewhat soft to the touch compared to marble or granite, and will effervesce (fizz) when dilute hydrochloric acid is dropped on it due to its calcite composition. Commonly found in areas with active or geologically recent hot springs, such as Italy (Tivoli), Turkey (Pamukkale), and the USA (Yellowstone).