A Practical Evaluation of Lab-Grown Gems During Torch and Silver Clay Firing

By India Stephens
Published by Gemdrop® Research
Testing conducted 11–12 July 2026

Introduction

One of the most common questions from jewellery makers is:

"Can this gem be fired directly in silver clay?"

Although recommendations exist for many gemstones, relatively little practical information is available for modern laboratory-grown materials such as synthetic corundum, moissanite and cubic zirconia.

To investigate this, jewellery artist India Stephens carried out a series of practical firing experiments using gems representative of those commonly used in silver clay jewellery.

The purpose was not to establish definitive engineering limits, but to evaluate how these materials behave under realistic studio conditions.

Materials Tested

The following gems were evaluated:

• Lab-grown Ruby (Flame Fusion Corundum)
• Lab-grown Pink Sapphire (Flame Fusion Corundum)
• Moissanite
• Ice Cut Cubic Zirconia

Test Method

Each gem was inspected before testing to ensure there were no obvious visible flaws.

The stones were then exposed to three increasingly demanding firing stages.

Stage 1

Medium torch flame

Exposure time:

15 seconds

Stage 2

Medium torch flame

Exposure time:

30 seconds

Stage 3

The gems were embedded into silver clay and fired in place for approximately 6 minutes, simulating a typical silver clay firing cycle.

Between each stage the gems were allowed to cool naturally for at least fifteen minutes before assessment.

Results

Individual Observations

Ruby

The ruby temporarily darkened during heating but returned to its original colour within approximately thirty seconds of cooling.

No fractures, chips or visible damage were observed.

Result: Excellent performance.

Pink Sapphire

The sapphire briefly appeared grey while hot before rapidly returning to its original colour after cooling.

No visible damage occurred.

Result: Excellent performance.

Moissanite

The moissanite remained unchanged during both torch heating stages.

Following firing within silver clay, however, the stone developed a brown/champagne appearance.

Although the finished gem remained structurally intact and could still be considered attractive, the colour change was considered unpredictable.

Result: Not recommended for fire-in-place work where colour consistency is important.

Cubic Zirconia

After approximately fifteen seconds a small corner fracture became visible.

The author believes this may have been associated with a pre-existing flaw, as no further physical damage developed during subsequent firing.

The colour changed progressively:

• Mid blue
• Dark blue
• Deep forest green

Following the initial colour shift, no additional change occurred during the six-minute firing cycle.

Although the colour changed substantially, the final appearance remained attractive.

Result: Potentially suitable where the expected colour change is acceptable.

Overall Conclusions

Excellent choices for fire-in-place jewellery

✅ Lab-grown Ruby (Corundum)

✅ Lab-grown Pink Sapphire (Corundum)

Both materials tolerated all firing stages exceptionally well and showed no permanent visible damage.

Use with caution

⚠️ Cubic Zirconia

The material remained structurally stable after an initial colour change, although colours may darken considerably depending on composition.

Testing individual gems before production is recommended.

Not recommended

❌ Moissanite

While structurally unaffected, prolonged firing produced an obvious colour change.

Jewellers requiring consistent appearance should avoid firing moissanite directly in silver clay.

Practical Recommendations

Based on this evaluation, the author recommends:

• Smaller gems (under approximately 5 mm) are generally easier to set securely in wet silver clay.
• Lab-grown corundum (ruby and sapphire) performed exceptionally well and would be used again for fire-in-place jewellery.
• Cubic zirconia may be suitable where colour changes are acceptable or desirable.
• Moissanite should generally be set after firing rather than fired in place.

A Note on This Evaluation

This article describes the observations from a practical workshop evaluation performed under one set of firing conditions.

Results may vary depending on:

• kiln or torch temperature
• firing duration
• heating atmosphere
• gem manufacturer
• individual gem characteristics

Jewellers should always perform their own testing before beginning production.

Many thanks to jeweller and author: India Stephens

Practical fire-in-place evaluation conducted independently using commercially available laboratory-grown gemstones.