In the world of fine gemstones, few topics generate as much confusion—or controversy—as treatments. Walk into any reputable jewelry store, and you'll encounter terms like "heated," "oiled," "enhanced" and "untreated" attached to gemstones with price tags that can vary dramatically. Understanding these distinctions is essential for making informed decisions about acquisitions.
The reality is straightforward: the vast majority of gemstones available today have undergone some form of treatment. This is standard practice in the industry, and proper disclosure should accompany each stone. The key lies in understanding what these treatments mean for beauty, durability and long-term value, and in finding knowledgeable dealers like M.S. Rau who go to great lengths to source exceptional untreated gemstones for discerning collectors.
The Spectrum of Natural vs. Enhanced
"Natural" means not laboratory-created—a natural ruby remains natural whether it's heated or untreated. "Untreated" indicates freedom from all enhancements, while "unheated" specifically refers to stones that haven't undergone thermal treatment, though they may have received other enhancements.
The distinction matters because untreated gemstones command premium prices due to their rarity. Fewer than one percent of all gem-quality rubies, for example, remain untreated, and these rarities seldom exceed two carats.
Emeralds
| (left) Untreated Emerald Ring, 4.62 carats. M.S. Rau. (right) Muzo Emerald and Argyle Pink Diamond Ring, 5.72 carats. Minor oil used. M.S. Rau. Sold. |
Emeralds present perhaps the most accepted case for enhancement in the gemstone world. Over 99 percent of emeralds receive clarity treatments, typically through oiling with cedar oil or resin filling. This acceptance stems from emerald's classification as a Type III gemstone—stones that are almost invariably heavily included due to their formation process.
The enhancement process involves filling surface-reaching fractures with oil that has a similar refractive index to the emerald itself, making the fractures less visible and improving the stone's overall clarity. This treatment is temporary; over time, the oil dries out and becomes visible, requiring re-oiling to maintain the stone's appearance.
Importantly, color remains the primary value determinant for emeralds, and this characteristic is rarely enhanced. The finest emeralds display their coveted vivid green through natural chromium and vanadium content, not artificial coloring agents. Untreated emeralds remain one of the most desirable gemstones today due to their rarity, especially in sizes exceeding one carat. These oil-free treasures command top prices on the market and remain highly desirable
Sapphires and Rubies
| (left) Raymond Yard Untreated Sapphire Ring, 3.79 Carats. M.S. Rau. (right) Color Change Sapphire Ring, 3.99 Carats. Heated. M.S. Rau. (sold). |
Heat treatment represents the most common and widely accepted enhancement for corundum, the mineral family that includes both sapphires and rubies. Over 95 percent of sapphires undergo heating at extremely high temperatures to enhance color and clarity, making unheated specimens exceptionally rare.
The heating process is permanent and stable; a heated sapphire will maintain its enhanced appearance indefinitely under normal wear conditions. The treatment doesn't compromise durability, making heated stones suitable for daily wear in engagement rings and other frequently worn pieces.
Certain colors, particularly the coveted royal blue associated with Kashmir sapphires, are nearly impossible to find in unheated stones. This rarity explains why unheated sapphires of fine color can command prices several times higher than their heated counterparts.
| (left) Raymond Yard Untreated Ruby Ring, 2.25 Carats. M.S. Rau. (right) Ruby Ring by Oscar Heyman, 6.13 Carats. Heated. M.S. Rau (sold). |
Ruby treatments can extend beyond heating. Fracture filling with lead glass can dramatically improve clarity but creates durability concerns, as the glass may be damaged during cleaning. More sophisticated flux healing uses high-temperature treatment to actually heal fractures within the stone, creating a more durable enhancement.
Diamonds
Diamond treatments, while less common than those applied to colored stones, address both clarity and color concerns. Laser drilling removes dark inclusions by creating microscopic channels for acids to remove or lighten them—a permanent treatment that gemological laboratories readily identify and note on certificates.
Fracture filling presents a different approach, using glass-like substances to improve apparent clarity. However, this treatment isn't permanent or stable and can affect the appearance of color, which explains why the Gemological Institute of America doesn't grade fracture-filled diamonds.
Color treatments range from surface coatings that can wear off to irradiation that permanently alters the atomic structure. High-pressure, high-temperature (HPHT) treatment can transform less conventionally appealing brown diamonds into more desirable colors—a stable process that major gemological laboratories accept for grading.
The Paraiba Tourmaline
| (left) Unheated Brazilian Paraiba Pendant, 23.50 carats. M.S. Rau. (right) Brazilian Paraiba Ring, 2.18 Carats. Minor heat. M.S. Rau. |
Paraiba tourmalines exemplify how rarity intersects with treatment acceptance. These stones, prized for their extraordinary neon blue glow caused by copper content, represent one of the gemstone world's most dramatic discoveries. However, over 99 percent undergo treatment to achieve their signature brilliance.
The rarity of untreated Paraiba tourmalines means they command astronomical prices when they appear at auction. Treated examples, while more accessible, still represent significant investments due to the stone's inherent scarcity and unique optical properties.
Tanzanite
| (left) Untreated Tanzanite Pendant, 104.39 Carats. M.S. Rau. (right) Emerald Cut Tanzanite Ring, 5.77 Carats. Heated. M.S. Rau. (sold) |
Virtually all tanzanite receives heat treatment to transform its natural brownish or yellowish appearance into the signature blue-purple that defines the variety. Interestingly, this same transformation occurs naturally within the earth through geological heating, making the distinction between natural and enhanced somewhat philosophical.
Found exclusively in Tanzania, tanzanite's extreme geographic limitation contributes to its desirability. Unheated specimens are 1,000 to 10,000 times rarer than diamonds, making them among the scarcest gemstones available to collectors.
Quick Guide to Other Gemstone Treatments:
Jadeite jade – Jadeite is often bleached with acid to remove an unwanted brown component from the material.
Pearls – All types of pearls are routinely bleached with hydrogen peroxide to lighten and improve their uniformity of color. They can also be dyed. Dye often improves the appearance of lower–quality natural and cultured pearls by enhancing their color. The majority of pearls are cultured (grown intentionally by humans), and true “natural” pearls are incredibly rare.
Topaz – Some colorless topaz is coated with metal oxides to create the appearance of a variety of different colors.
Coral – Some black coral has been reported as bleached and then coated with relatively thick layers of artificial resin with the goal of protecting the coral and intensifying its color. Once bleached, it can also be dyed.
Quartz – Occasionally, quartz is coated with metal oxides to create colors rarely seen in natural quartz.
Amber – When amber is submerged in hot oil (linseed oil, for example), its inherent body color can darken, and the material can take on a clearer appearance. The hot oil can also cause the material to develop a series of spangled, glittery inclusions.
Amethyst – Heating amethysts can remove unwanted brownish inclusions in some amethysts or lighten the color of overly dark stones.
Citrine – Some forms of amethyst can be heated and turned into citrine.
Making Informed Decisions
Whether you're drawn to the pure beauty of an untreated Kashmir sapphire or the enhanced brilliance of a heated ruby, knowledge remains your most valuable tool in navigating the complex world of gemstone treatments. Each stone tells a story—some untouched wonders of nature, others elevated by human intervention—and understanding these nuances enriches the collecting experience immeasurably.
Don’t forget to explore our many offerings in our high jewelry, fine jewelry and everyday jewelry collections.
Sources:
GRS GemResearch Swisslab AG. “Gemstone Enhancement Disclosure.” Accessed September 11, 2025. Source.
Smith, Christopher P., and Shane F. McClure. “Chart of Commercially Available Gem Treatments.” Gems & Gemology38, no. 4 (Winter 2002): 294–300. Gemological Institute of America.
Weldon, Robert. “An Introduction to Gem Treatments.” Gemology Institute of America.
