Ultrasonic Cleaners: When to Use Them, When to Avoid Them

The ultrasonic cleaner is one of the most useful tools in any jewellery workshop — and one of the most dangerous if used without knowledge. In the right hands, on the right stones, it restores brilliance in minutes that would take hours with a brush and soap. In the wrong hands, on the wrong stones, it can fracture an emerald, destroy pearl nacre, expand the fractures in a tanzanite, or drive moisture into a porous opal. The technology is impressive, the results are dramatic, and the failure modes are unforgiving. Understanding exactly when and how to use an ultrasonic cleaner is essential professional knowledge for every jeweller.

This article covers the science behind ultrasonic cleaning, the conditions under which it is safe, the gem types and treatments that must never go into an ultrasonic, and the protocols for using this tool correctly.

How Ultrasonic Cleaners Work

An ultrasonic cleaner consists of a tank filled with a cleaning solution (usually water with a cleaning additive) and a transducer that vibrates at ultrasonic frequencies — typically between 20,000 and 400,000 Hz, well above the range of human hearing. The vibrating transducer creates rapid pressure waves in the liquid that produce microscopic cavitation bubbles.

Cavitation occurs when a rapidly oscillating pressure wave causes dissolved gases and vapour to form tiny bubbles — and then causes those bubbles to collapse violently. The collapse of each bubble releases a small but intense burst of energy — a micro-jet of liquid moving at high velocity. On the surfaces of objects placed in the cleaner, these micro-jets physically blast away deposits: grease, dirt, soap residue, skin oils — even material lodged in the tightest corners of a complex setting.

The cleaning action is the physical energy of cavitation, not chemical action. The cleaning solution may contain surfactants or ammonia to help loosen deposits, but the primary mechanism is mechanical. This is what makes ultrasonic cleaning so effective — and what makes it damaging to vulnerable materials.

When Ultrasonic Cleaning Is Safe

The ultrasonic cleaner is safe for gemstones that meet all three of these criteria:

Criterion 1: High Hardness and Toughness

The micro-jets of cavitation impact the gem surface repeatedly during a cleaning cycle. A hard, tough gem can absorb this without damage. The benchmark safe gems are diamond and untreated corundum (ruby and sapphire): hardness 9–10, excellent toughness, no fracture fill. Spinel (hardness 8, excellent toughness) is also generally safe. Most garnets (hardness 6.5–7.5, good to excellent toughness) can be safely ultrasonicated if they show no significant fractures.

Criterion 2: No Fractures or Inclusions That Reach the Surface

Even a hard gem can be damaged by ultrasonic cleaning if it has surface-reaching fractures. The cavitation energy is concentrated at discontinuities and boundaries — including fractures. A fracture in a diamond that runs to the surface can propagate under repeated ultrasonic exposure. A deep feather in a sapphire that barely reaches the surface can be opened further. Pre-cleaning inspection under magnification is essential to identify and assess any surface-reaching fractures.

Criterion 3: No Treatments That Are Damaged by Mechanical Energy or Moisture

Treatment status is often the determining factor for borderline cases. An emerald in a bezel setting with excellent apparent clarity might be tempting to ultrasonicate. But virtually all commercial emeralds are oiled or resin-filled, and the mechanical energy and moisture of the ultrasonic bath will force the fill out of the fractures — leaving the stone looking worse than before, often dramatically so. Always assume an emerald is treated unless you have laboratory certification stating otherwise.

Gemstones That Must Never Enter an Ultrasonic

These gem types and treatments are categorically unsafe for ultrasonic cleaning:

Emerald: treated with oil or resin in virtually all commercial specimens; fill is driven out

Opal: poor toughness, sensitive to temperature change and moisture; cavitation can fracture and damage

Pearl: nacre layers may be physically loosened; cord on strands is weakened; nacre bond with nucleus can fail

Tanzanite: poor toughness (cleavage), combined with temperature sensitivity; cavitation-induced fracturing is a real risk

Coral: carbonate structure damaged by mechanical energy and by acidic cleaning solutions sometimes used in ultrasonic tanks

Amber and organic gems: soft, physically vulnerable to cavitation energy

Glass-filled (lead glass) rubies: glass fill can be dislodged or damaged

Fracture-filled diamonds: glass fill can be damaged by prolonged ultrasonic exposure

Any stone with a loose setting: cavitation loosens prongs and settings; a loose stone will become a lost stone

Kunzite: colour sensitive to heat generated in extended ultrasonic cycles

Turquoise and lapis lazuli: porous, absorb cleaning solution, damaged by prolonged moisture

Safe Use Protocols

Pre-Cleaning Inspection

Before placing any piece in an ultrasonic, examine it under 10x magnification. Check: prong tightness (tap gently and listen for rattling), surface-reaching fractures in any stone, presence of foil-backed stones (antique pieces sometimes use foil behind stones to enhance colour — ultrasonic destroys this), and any unusual inclusions or surface features that might indicate treatment.

Cleaning Solution

Use the cleaning solution recommended by the manufacturer, or plain water for very sensitive pieces. Ammonia-based solutions work well for diamonds and gold. Avoid acidic solutions, which can attack some stone surfaces and most metals other than platinum and fine gold. Change the cleaning solution regularly — dirty solution redeposits material rather than removing it.

Cycle Duration

Most pieces need only one to three minutes in an ultrasonic cleaner. Extended cycles increase the risk of damage. For borderline cases — a diamond with a small included feather, for example — use a shorter cycle and inspect mid-cycle. Never leave pieces unattended in an ultrasonic for extended periods.

Temperature

Many ultrasonic cleaners heat the solution, which improves cleaning action but increases thermal stress risk. For gems at the margins of safe use, room-temperature solution is safer. Never use ultrasonic cleaners with very hot solution on any coloured gemstone.

Post-Cleaning Rinse

Always rinse pieces thoroughly under clean water after ultrasonic cleaning to remove all cleaning solution residue. Residual cleaning solution can continue to act on stone and metal surfaces after removal from the cleaner. Pat dry with a soft cloth; allow to air dry completely before storage.

Key Takeaways

Ultrasonic cleaners use cavitation — microscopic bubble collapse — to physically blast deposits from surfaces.

Safe for: hard, tough, untreated gems in secure settings (diamond, unheated ruby/sapphire, most garnets).

Never use for: emerald, opal, pearl, tanzanite, coral, amber, glass-filled rubies, any loose setting.

Pre-cleaning inspection is mandatory: check prong security, surface fractures, and treatment status.

Shorter cycles, room-temperature solution, and careful rinsing reduce risk in borderline cases.

“When in doubt, leave it out” — a soft brush and warm soapy water is always the safe alternative.