Independent Thermal Testing of Gemdrop® Lab-Grown Ruby & Sapphire

A Scientific Investigation into Fire-in-Place Gem Performance

Research collaboration: Gemdrop® and Emma Gordon, Silver Clay School

Can a beautiful, precision-cut ruby or sapphire survive being fired directly inside metal clay at temperatures approaching 1,000°C?

For jewelry makers working with precious metal clay, this is an important question. Traditionally, firing gems in place carries significant risks, including thermal shock, cracking, clouding, discoloration, and adverse reactions between the gem and the surrounding metal.

At Gemdrop®, our approach is rooted in mineralogy and materials science. We believe that the physical properties of lab-grown crystalline gems make certain materials promising candidates for high-temperature jewelry manufacturing. However, theoretical stability alone is not sufficient to establish suitability for real-world firing processes.

To investigate this, Gemdrop® supplied lab-grown corundum gems for practical evaluation by Emma Gordon of Silver Clay School. We contacted Emma as an independent specialist, and at the time of email contact with Emma we had no prior relationship with Emma, we hope that our impressive, and thermally stable, gems will help us build a lasting business relationship and freindship :)  

The results were highly encouraging: all three tested colors—red, blue, and white—survived the reported firing trials without visible deterioration.

You can purchase the gems used in this testing here.

Why Corundum?

Ruby and sapphire are varieties of the mineral corundum, crystalline aluminum oxide (Al₂O₃).

Lab-grown corundum possesses the same fundamental chemical composition and crystal structure as naturally occurring corundum. Its high hardness, approximately 9 on the Mohs scale, and high melting point, approximately 2,050°C, make it an interesting candidate for high-temperature applications.

However, a high melting point does not automatically guarantee successful kiln firing. Thermal expansion, internal strain, inclusions, color-producing elements, heating and cooling rates, and interactions with surrounding materials can all affect the outcome.

For this reason, we sought practical testing under genuine jewelry-making conditions.

The Testing Programme

Emma evaluated red, blue, and white lab-grown corundum using butane torch firing and kiln firing in three different metal-clay systems.

The tests included:

• Fine silver clay (Art Clay 999)

• Copper clay (Art Clay)

• Bronze clay (Prometheus)

• Direct butane torch exposure

• Multiple firing cycles

• Post-firing tumble polishing

Importantly, several gems were subjected to more than one firing process, providing additional observations of their ability to withstand repeated thermal exposure.

Results: High-Temperature Firing Performance

All three tested colors—red, blue, and white—performed successfully across the reported metal-clay trials.

1. Silver Clay: Two Hours at 900°C

Red, blue, and white corundum gems were set directly into fine silver clay and fired at 900°C for two hours.

All three colors emerged without reported clouding, cracking, or adverse reactions with the silver.

This is a particularly encouraging finding for silver-clay jewelry makers interested in incorporating lab-grown ruby and sapphire into their designs.

2. Copper Clay: High-Temperature Carbon Firing

Copper clay presented one of the most demanding conditions in the testing programme.

Following an initial burnout stage, the gems were fired in carbon at a reported sintering temperature of 982°C for two hours.

Despite the prolonged high-temperature exposure, all three tested colors retained their appearance without reported damage or adverse interaction with the surrounding copper clay.

3. Bronze Clay: Successful Firing in Carbon

Red, blue, and white corundum gems were also incorporated into bronze clay and subjected to a two-stage firing process, including a reported 820°C carbon-firing stage.

Again, Emma observed no clouding, cracking, discoloration, or adverse reaction with the clay.

Beyond a Single Firing: Repeated Thermal Exposure

One of the most interesting aspects of the investigation was that testing did not stop after a single successful firing.

Some gems were first exposed directly to a butane torch, subsequently set into silver clay and torch-fired again, and then subjected to further kiln firing.

Other samples incorporated into base metal clay underwent additional firing cycles alongside other projects.

Emma reported that the gems continued to perform well.

Following firing, the metal settings were also tumble-polished, with no adverse effects observed.

These observations suggest that the tested samples may be suitable for certain jewelry-making workflows involving more than one heating and finishing stage.

An Important Finding for Blue Gems

Blue gems can be particularly challenging for metal-clay artists because certain materials and color-producing mechanisms are susceptible to changes during heating.

In this investigation, the tested blue lab-grown corundum retained its appearance throughout the reported firing trials, including high-temperature firing in carbon.

This is a promising result for makers seeking reliable blue gems for fire-in-place designs.

Nevertheless, the findings apply to the specific samples and conditions evaluated. They should not be interpreted as a guarantee that every blue gem, or every formulation of lab-grown sapphire, will behave identically.

What These Results Mean for Jewelry Makers

The investigation provides encouraging evidence that selected Gemdrop® lab-grown ruby and sapphire gems can tolerate firing schedules used in fine silver, copper, and bronze metal clay.

For jewelry makers, the potential benefits include a wider selection of gem colors, greater design flexibility, and the possibility of setting certain gems directly into unfired clay rather than incorporating them after firing.

However, the results are best understood as a successful initial practical evaluation, not universal certification of fire-in-place suitability.

Different gem batches, color varieties, dimensions, cuts, firing atmospheres, clay formulations, and cooling procedures may produce different outcomes. Further controlled testing would be needed to quantify failure rates and establish reproducibility across a broader range of samples.

Buy the exact gems used in this testing here.

Independent Testing and Scientific Transparency

At Gemdrop®, we believe that beautiful gems should be supported by meaningful scientific evidence.

This investigation combines Gemdrop®'s mineralogical perspective with Emma Gordon's practical expertise in metal-clay firing.

We are grateful to Emma for her careful observations, multiple firing trials, and willingness to evaluate the materials under genuine working conditions.

The results support further exploration of lab-grown corundum for fire-in-place jewelry applications and provide a useful starting point for makers seeking evidence-based guidance.

What's Next?

Lab-grown corundum is only one part of the Gemdrop® collection.

Other crystalline materials, including spinel and moissanite, present interesting opportunities for further thermal-performance investigations. Their suitability must be evaluated independently, rather than inferred from the results for corundum.

Future testing could also investigate additional sapphire colors, different firing atmospheres, thermal cycling, and the effects of gem size and cut geometry.

Our objective is straightforward: to help jewelry makers choose beautiful, scientifically verified gems with greater confidence.

BIG GEMS. ETHICAL SPARKLE. SCIENTIFICALLY VERIFIED.

Research note: This article summarizes the practical observations reported by Emma Gordon in “Gemdrop Testing in Metal Clay Results – Lab Corundum (Al₂O₃).” Note that results are qualitative and specific to the tested samples and firing conditions.