Certain materials are either impossible to cut with a waterjet or pose significant dangers that make the process inadvisable.

Tempered glass is impossible to cut with a waterjet. Tempered glass is manufactured with high internal compressive stress and tensile stress. Any scratch or localized cut through the surface releases this stored energy instantly, causing the entire sheet to shatter into thousands of small pieces. The waterjet stream, even at low pressure, creates exactly the type of surface penetration that triggers complete failure.

Laminated safety glass with polyvinyl butyral interlayers is not impossible but produces unusable results. The waterjet cuts through the glass layers but cannot cleanly sever the plastic interlayer without delamination. Water migrates between the layers, and the cut edges separate. The resulting part is no longer a bonded safety glass unit.

Diamond is effectively impossible to cut with a pure waterjet. Diamond is the hardest known material. An abrasive waterjet using garnet can erode diamond very slowly, but the process is impractical. The garnet abrasive is softer than diamond, so removal rates are negligible. A waterjet would require hours to cut even a thin diamond wafer.

Certain explosive and reactive materials are extremely dangerous to cut with a waterjet. The high-pressure water stream can generate heat through friction as the jet erodes the material. Some sensitive explosives, such as lead azide or acetone peroxides, can detonate from friction or impact. Water immersion does not guarantee safety, as many explosives contain their own oxidizers and can react underwater. Similarly, reactive metals like sodium, potassium, and lithium explode on contact with water. Cutting these with a waterjet triggers an immediate violent reaction.

Pyrophoric materials, including finely divided iron or certain metal hydrides, ignite spontaneously in air. The waterjet spray does not prevent ignition because the cutting process exposes fresh unoxidized surfaces that can reach ignition temperature from friction.

Some composite materials containing reactive binders or fillers are hazardous. Boron carbide composites and certain magnesium alloys generate hydrogen gas when wet. Confined hydrogen can ignite from static charges generated by the waterjet stream.

Finally, materials that produce toxic airborne compounds are dangerous without specialized containment. Cutting beryllium copper or beryllium alloys releases beryllium particles into the water mist. Inhaling beryllium causes chronic lung disease. Cutting asbestos composites releases carcinogenic fibers into the slurry. These materials are cut only in sealed, negative-pressure waterjet systems with full containment and hazardous waste disposal.