Travertine are particular carbonate rocks, easily distinguished from other limestone for their different appearance: in fact, they have a very compact paste made up of grains so fine they are invisible to the naked eye and feature vacuoles, natural holes that typify the surface, known in sector jargon as taroli and sometimes entirely covered with tiny crystals. This aspect is quite visible if the material is cut al contro (as is generally the case with travertine), with the vacuoles riddling the surface and only sometimes showing the Barcelona. The Sant Jordi sports arena, seating 17,000, designed by A. Isozaki, with a dark, symmetrical roof and travertine-clad entrance S4 stratified or zoned structure of the material, often visible only under close observation. Even in the cut verso (with the grain) the vacuoles are visible, but less. evident: the material reveals the circular and nearly circular shapes typical of concretion, in some varieties quite evident and in others merely hinted at. The vacuolar nature of travertine is a typical characteristic, although in reality not all travertine have cavities, at least not in an evident fashion. Travertine are often used filled-in, and in this case it is the particular non-crystalline, sometimes porous, look of the material that distinguishes it. In addition, the material always reveals concretion motifs, most often in the form of small spheroid shapes and frequently along the stratification planes which, unlike those in normal sedimentary rocks, are not linear but very wavy, variegated, thanks also to these little round forms that give further motion to the pattern. The particular appearance of travertines that differentiates them from other limestones - and commercially sets them into a class by themselves - is due to their different origin; in fact, these are concretion rocks, similar to alabasters and stalactites, while the other carbonate rocks are generally marine in origin. This means that travertines were formed mostly near the surface, while rocks in general were formed at great depths, often subjected to pressures and tectonic thrusts that provoked phenomena of re crystallization and rock hardening. Furthermore, travertines are geologically recent rocks .