Cave Pearls Uncover Jerusalem’s Ancient Water Management Techniques

Ancient hydraulic systems, essential to the survival and development of civilizations, fascinate with their complexity and resilience. A recent study, led by researchers at the Hebrew University of Jerusalem and published in the journal Archaeometry, has uncovered rare cave pearl formations in the Ain Joweizeh Spring Tunnel near Jerusalem.

These beads, incorporating archaeological artifacts such as pottery and plaster fragments, reveal valuable information about hydraulic practices, cultural exchanges and environmental developments in the region since the Iron Age. This exceptional discovery illustrates how natural structures can become unexpected witnesses of the complex interactions between human ingenuity and geological processes. It also sheds light on the social and economic history of Jerusalem and its surroundings.

Cave pearls: a natural rarity

Cave pearls are special geological formations. They are produced by a precise process where concentric layers of calcite accumulate around an initial core. The latter may include a grain of sand or a fragment of organic matter. The phenomenon requires specific conditions, including high saturation of calcium carbonate in the water, constant movement to prevent adhesion of the beads to the substrate, and shallow water depth.

This type of formation remains rare, particularly in the southern Levant, where their documented presence remains exceptional. Their growth is generally rapid on the geological scale. It offers a unique opportunity to observe environmental and hydrological dynamics over relatively short periods.

(a) Sites in Israel where cave pearls have been found (black circles). (b) Detailed map of the Jerusalem Hills showing the Joweizeh Spring (ST) tunnel (red arrow). © A. Yechezkel et al., 2024

In the Ain Joweizeh tunnel, located near Jerusalem, researchers have uncovered 50 of these formations. Some incorporate fragments of ancient pottery or plaster. These artifacts, serving as cores, constitute a world first. The dating of the encrusted materials made it possible to trace the history of human activities and environmental transformations in the region. This debris could have been left in the tunnel, potentially during renovations or passages. As Dr. Azriel Yechezkel pointed out, this phenomenon illustrates the impact of human activities on geological processes once perceived as entirely natural.

A sophisticated hydraulic architecture

The Ain Joweizeh tunnel, housing these pearls, represents an impressive engineering achievement of Iron Age II. The inhabitants of the time designed it to capture and channel water from an underground aquifer. This structure, 232 meters long, includes a section cut directly into the bedrock. The other section is built with cut stones, demonstrating advanced know-how.

Engineers of the time also integrated water flow management systems. We thus find internal plaster channels. They had to maximize the efficiency of hydraulic transport. These characteristics thus testify to the complexity of the infrastructure necessary to meet the water needs of the agricultural and royal communities of Jerusalem and its surroundings. According to analyses, these installations were used and maintained for several centuries, going through periods of crisis and redevelopment.

Researchers identified Hellenistic renovations, including the addition of plaster to certain sections of the tunnel. This seems to correspond to a phase of economic and agricultural revitalization of the region. The plaster fragments embedded in the cave beads, carbon-14 dated, suggest that these renovations aimed to adapt the infrastructure to the growing needs of the Refaïm valley. It was a crucial agricultural center at the time.

Indeed, this valley, with its numerous terraces and intensive crops, provided on the one hand foodstuffs. It also played a key role in supporting Jerusalem's growing urban population. These renovations demonstrate the strategic importance of water management in a region where water resources were as precious as they were ephemeral, dictated by unpredictable seasonal cycles.

Pearls revealing cultural exchanges

Cobalt-enriched pottery fragments from cave beads reveal extensive Hellenistic trade networks and cultural exchange. Indeed, cobalt is an expensive pigment and not native to the region. It was commonly found used in luxury ceramics produced in Cyprus or Ephesus. Two major craft centers in the Eastern Mediterranean. These pottery fragments, identified as belonging to imported lamps, not only reflect extensive trade flows.

These fragments also demonstrate consumption reserved for a social elite capable of obtaining these precious objects. The use of these artifacts in the Ain Joweizeh tunnel reinforces the idea that the site served a utilitarian and symbolic function. This perhaps marks strategic status or importance in the management of water for agricultural and residential purposes.

Archaeomagnetic analysis of the fragments made it possible to date these objects and identify key periods of use of the tunnel, from the Iron Age II to the Roman and Byzantine periods. The discovery of an intact lamp, dated to the 3rd–4th centuries, testifies to the persistence of human activities in the tunnel well after its initial construction. This confirms that the site, although ancient, has been regularly redeveloped to meet the changing needs of the surrounding communities. These imported objects, in addition to their practical function, tell a story of cultural connections, trade, and social organization where control of water played a central role in the development and prosperity of Jerusalem.

A surprising rediscovery

The rediscovery of the cave pearls in the Ain Joweizeh tunnel in 2017 is not only an archaeological surprise, but also raises important questions about the preservation and management of historic infrastructure. Their grouping in a side section, apparently blocked for centuries, suggests that human interventions took place to reorganize the space.

(a) Cave bead sample before analysis. (b) J-10 bead with a pottery sherd core, showing measurement methods. (c) Beads formed on a plaster, containing charcoal used for 14C dating. (d) Beads used for stable isotope measurement. (e) J-25 pearl formed on limestone. (f) Three slabs along the main axis of bead J-28. A secondary core made of a piece of plaster (3, red arrow). (g) Largest pearl, J-29. © A. Yechezkel et al., 2024

The case of the Ain Joweizeh tunnel reminds us that the buried history of ancient infrastructures continues to shape our understanding of past societies. These structures, designed to meet practical needs, have become living archives where nature and culture meet. The future of these sites, as objects of study and preservation, will depend on the balance between scientific exploration and heritage preservation.

But also geopolitical hazards. Future research may reveal more artifacts, but also enrich our understanding of human practices in the context of the environmental challenges of their time. This would thus offer useful perspectives for current resource management issues.

Source: Azriel Yechezkel et al., “Dating an ancient spring tunnel using archaeological artifacts functioning as nuclei of cave pearls”, Archaeometry, 2024