If you have looked closely at rubies or sapphires, you will have seen the words heated and unheated (sometimes written as no heat). The difference affects both rarity and price, so it is worth understanding before you buy.
What heat treatment is
Rubies and sapphires are both corundum. Heating corundum in a controlled furnace is a long-established practice that can deepen or even out colour and dissolve fine needle-like inclusions, making a stone brighter and clearer. The great majority of rubies and sapphires sold today have been heated in this way.
Conventional heating is widely accepted in the trade and is considered stable: the improvement does not fade with normal wear. It is different from other treatments, such as filling fractures with lead glass, which are less durable and priced very differently. Whatever the treatment, it should always be disclosed.
Why unheated stones are valued more
A ruby or sapphire that shows fine colour without any heating is rarer, because nature alone produced it. For stones of comparable size, colour and clarity, unheated material generally commands a significant premium over heated material.
How a laboratory tells the difference
Gemmologists examine a stone's inclusions under magnification. Heat leaves characteristic signs, such as altered needles or tension halos around crystals. Where none are found, a report will typically state that there is no indication of heating. This is a conclusion based on evidence, which is why an independent report matters for any unheated stone.
How we describe our stones
Every Silva Jewellers listing states the treatment of each gemstone where it is known, and names the laboratory where a report exists. You can browse our unheated rubies and sapphires and unoiled emeralds, or see all rubies, blue sapphires and pink sapphires.
For more on treatments, karats and lab reports, read our Gold & Gemstone Guide.