## 1 Introduction  
Natural stones have historically played a central role in architecture, not only due to their availability and esthetic appeal but also because of their durability and environmental performance [1, 2, 3]. Their presence in both culturally significant buildings and contemporary constructions underlines their relevance as a building material [4]. However, stone is not immune to degradation. A wide range of deterioration mechanisms can affect its integrity, depending on environmental and contextual factors [5]. For instance, surface flaking has been linked to high-temperature exposure, but similar patterns have also been observed in heritage contexts unrelated to fire [6], highlighting the complexity and diversity of these processes. Over time, deterioration can be triggered by multiple factors, including pollution, biological colonization, salt crystallization, mechanical stress, and increasingly, climate-induced hazards such as wildfires [7, 8, 9, 10, 11, 12]. Although stone itself is a noncombustible material, climate change alters the surrounding environment in ways that exacerbate its decay. For example, elevated temperatures and reduced humidity promote the frequency and intensity of wildfires, which can lead to irreversible damage in stone structures, particularly in heritage contexts [13; 14; 15; 16]. The severity of fire-induced damage depends on the stone's lithological properties, like mineralogical composition, grain size, fabric, and porosity, all of which influence its thermal response [17; 18]. Carbonate stones such as limestones and marbles are especially sensitive due to the anisotropic thermal behavior of calcite, their dominant mineral phase. Calcite undergoes anisotropic expansion from approximately 200-300 \({}^{\circ}\)C, leading to transgranular and intergranular microcracking, with more severe transformations including decarbonation and porosity increase, occurring above 600 \({}^{\circ}\)C [19; 20; 21; 22; 23]. These thermal alterations compromise both mechanical strength and water-related properties such as capillary absorption [24; 25]. Water plays a critical role in stone deterioration. Capillary uptake introduces moisture into the porous network, triggering degradation processes such as freeze-thaw cycles, salt crystallization, swelling of clay minerals, and enhancing biological colonization [26; 27; 28; 29]. The extent and severity of these processes are closely related to stone's internal structure, particularly its porosity, fissure network, and mineralogical composition. When stones are exposed to high temperatures, as in fire scenarios, their microstructure can undergo significant transformations, such as pore enlargement and crack formation, creating new porosity structures that further influence their vulnerability to moisture-related decay [30; 31]. Assessing these internal changes is essential for understanding post-fire behavior, especially in complex lithologies where traditional visual inspection may not reveal the full extent of damage and in situations where water can be present. To address this challenge, non-destructive testing (NDT) techniques have emerged as valuable tools for evaluating stone condition without compromising material integrity. Their application is particularly relevant in heritage conservation and construction, where invasive methods are often restricted [32]. Among the various NDT tools available, ultrasonic pulse velocity (UPV) and Leeb hardness (HL) stand out due to their practicality, sensitivity to internal changes, and potential for correlation with key petrophysical properties. UPV has shown strong correlations with the presence and propagation of internal fissures, porosity, and early-stage weathering in carbonates [33; 34; 35]. HL, originally developed for metal materials, has been adapted for use on rocks and demonstrated potential as a predictor of uniaxial compressive strength (UCS) in various lithologies [36; 37; 38; 39; 40; 41; 42].