Zimmi Diamonds

A Premium the Trade Recognizes

Among yellow diamonds, there is a small, much-sought group known by the name of the place they come from: Zimmi, in southern Sierra Leone. These are Type Ib diamonds — among the rarest of all natural diamonds — and at their best they show some of the most intense yellow color found in nature.

Auction houses and dealers regularly describe top Zimmi stones as commanding prices up to, and sometimes more than, double those of other Fancy Vivid yellows. There is no published price index for Zimmi specifically, so that premium is a trade observation rather than a fixed figure; it varies with size, color, clarity and provenance.

Geographic Origin

Zimmi diamonds are recovered from alluvial (riverbed) deposits around the town of Zimmi in southern Sierra Leone, in the rainforest near the Mano River, which marks the border with Liberia. The Natural Diamond Council singles out Zimmi as a source of particularly vivid yellow gems, and within the trade "Zimmi" has become a byword for exceptional yellow color.

The Type Ib Difference

Zimmi diamonds contain nitrogen in its rare isolated form — single nitrogen atoms each replacing one carbon atom — which classifies them as Type Ib. GIA research notes that Type Ib diamonds make up less than 0.1% of the world's natural diamonds. In practice most Zimmi stones are a mixed type, described by GIA scientists as Type Ib-IaA: some of their nitrogen is still isolated, and some has begun to pair up. The isolated nitrogen is what produces their strong yellow. For how nitrogen and other trace elements create color, see The Science of Diamond Color.

Why Type Ib Is So Rare

At the temperatures found deep in the mantle, isolated nitrogen atoms in diamond migrate and pair up over geological time, turning Type Ib into the far more common Type Ia. Because most diamonds are one to three billion years old and spent that time hot, almost all of them have aggregated nitrogen. Isolated nitrogen survives only in diamonds that were kept unusually cool, or brought to the surface unusually soon.

The Zimmi Exception

GIA research scientist Dr. Karen Smit and colleagues dated sulphide inclusions inside Zimmi diamonds to about 650 million years ago — a time of subduction and mountain building along the edge of the West African craton. The kimberlites in this region erupted much later, around 150 million years ago. For isolated nitrogen to survive that half-billion-year wait, the diamonds must have been moved to shallower, cooler parts of the mantle soon after they formed. The study concludes they cannot have spent long above about 850°C, and estimates they rested at roughly 650–700°C, some 60–90 kilometers down.

The same tectonic journey left its mark inside the stones: GIA imaging shows deformation lamellae in Zimmi diamonds, and the resulting defects combine with isolated nitrogen to produce their color range.

Intensity the Trade Talks About

Isolated nitrogen absorbs blue light very efficiently, and the finest Zimmi yellows show a depth of color that auction houses and dealers describe as unrivalled among yellow diamonds. The best stones reach Fancy Vivid — the top intensity grade on the GIA fancy color scale.

Not every Zimmi diamond is vivid. GIA's scientific studies of Zimmi material describe a range from brownish-yellow to greenish-yellow; it is the saturated, pure-to-orangy yellows that carry the name's reputation.

Color Characteristics

• Intense yellow — Phillips has compared the color of fine Zimmi stones to "the golden rays of the sun"
• Orange modifier — Some Zimmi stones grade orangy-yellow or orange-yellow; auction houses have highlighted a "tangerine" tinge as characteristic (see orange and other fancy colors)
• Face-up appearance — As with all fancy colors, even distribution and a well-planned cut matter as much as the grade on the report

"Canary" is the older informal term for vivid yellow. Within the trade, "Zimmi yellow" has come to mean the most saturated end of that range — but the name describes an origin, not a GIA grade.

Crystal Shape

Zimmi rough is typically resorbed — smooth and rounded, often dodecahedral in form — rather than sharp-edged. Rounded rough can be easier to plan for yield, though weight retention still depends on each stone's shape, inclusions and the cut chosen to maximize color. Planning rough for color is the work of our diamond cutting services.

Typical Inclusions

Zimmi diamonds are known for abundant sulphide inclusions, sometimes with small graphitized fractures around them. These inclusions are what allowed scientists to date the diamonds; in well-selected stones they need not affect beauty.

Small Stones, Serious Value

Zimmi production is dominated by small stones. Dealers report rough rarely larger than about four carats, and most polished Zimmi diamonds weigh under a carat. When Tiancheng International offered a 6.70-carat Zimmi yellow in 2020, the auction house noted that Zimmi yellows over five carats are exceedingly rare.

No reliable public statistics exist for Zimmi output by size, so we do not publish percentages. What collectors can rely on is the pattern: rarity climbs sharply above one carat, and stones above two or three carats are seldom offered.

How the Premium Shows Up

Phillips has written that Zimmi diamonds often command more than double the price of other vivid yellow diamonds, and dealers commonly cite premiums of up to double. The size of the premium is not fixed: it depends on color, size, clarity, matching (for pairs) and how convincingly the origin is documented.

Retail: smaller Zimmi stones are commonly offered by specialist jewelers at around $20,000–$30,000 or more per carat. Large, top-color stones at auction have reached many times that.

Notable Auction Lots

• The Oriental Sunrise (Christie's Geneva, May 2016) — a pair of Fancy Vivid Orange-Yellow ovals of 12.20 and 11.96 carats, set as earrings with white diamonds, sold for about $11.6 million. Dealers who attribute the pair to Zimmi calculate roughly $470,000 per carat for the yellow stones. The same earrings were offered again at Christie's Hong Kong in October 2024 and sold for HK$62.02 million (about $8 million).
• 10.30-carat Fancy Vivid Orangy Yellow, internally flawless — offered at Christie's Geneva in November 2016 with an estimate of $2–3 million (about $194,000–$291,000 per carat).
• 6.70-carat Fancy Deep Yellow, VS1, from Zimmi — offered by Tiancheng International, Hong Kong, in November 2020 with an estimate of HK$3.5–5.5 million (about US$450,000–$705,000), alongside a pair of Zimmi earrings of 3.03 and 3.01 carats estimated at up to $320,000.

Estimates are not sale prices, and the 2024 resale of the Oriental Sunrise is a useful reminder that even exceptional stones can realize less on a later sale. Top Zimmi yellows sit in the same conversation as fine pink and blue diamonds at auction — a notable position for yellow — though per-carat prices for the finest pinks and blues remain higher.

What the Index Data Does — and Does Not — Show

The Fancy Color Research Foundation (FCRF) tracks prices for the main fancy colors. Over the twenty years to 2025, its yellow index rose about 49%, against roughly 242% for blue and 394% for pink; the fancy color category as a whole grew at a compound annual rate of about 5.7%.

There is no separate published index for Zimmi diamonds, so we do not quote a Zimmi return figure. The premium is visible in individual sales and dealer pricing rather than in an index. Past performance does not guarantee future results. For the wider picture, read our Diamond Investment Guide.

What Underpins the Premium

• Scientific rarity: Type Ib diamonds are a fraction of a percent of natural diamonds
• A single, small source: alluvial workings rather than a large industrial mine
• Collector recognition: the Zimmi name is well known among colored diamond buyers
• A remarkable geological story: documented by GIA research
• Origin as a value factor: as with Kashmir sapphires or Golconda diamonds, provenance can add value beyond grade — when it can be documented

Verification of Zimmi Origin

No single test proves Zimmi origin. A credible attribution combines several lines of evidence:

• Diamond type: spectroscopy confirming isolated nitrogen (Type Ib, typically Ib-IaA)
• Inclusions: sulphide inclusions consistent with published Zimmi material
• Color and appearance: consistent with known Zimmi stones
• Documentation: a chain of custody back to Sierra Leone
• Expert evaluation: assessment by specialists experienced with Zimmi diamonds

Important note: a standard GIA grading report does not state a diamond's mine of origin, so it will not say "Zimmi." The attribution rests on documentation and expert evaluation, which is why it should always be supported in writing. See Grading & Lab Certification for what a report does and does not state, and our authentication and verification standards.

Responsible Acquisition

Zimmi diamonds come largely from small-scale alluvial mining. Sierra Leone participates in the Kimberley Process, and rough diamonds exported from the country must travel with Kimberley Process certification.

Small-scale mining supports many families in the region, and standards on the ground vary. That is why we acquire Zimmi stones only through documented, traceable channels, and ask for the paperwork to prove it.

Supply Constraints

• Small-scale alluvial mining produces irregular supply
• Most production is under one carat
• Stones over two carats appear infrequently on the market
• Large, Fancy Vivid, high-clarity Zimmi stones are genuinely rare events

Where They Trade

The most important Zimmi stones tend to surface at major auction houses in Geneva and Hong Kong, while smaller stones move through specialist colored diamond dealers. Buyers in Asia have been particularly active in recent colored diamond sales. As with Kashmir sapphire, the Zimmi name carries an origin premium — provided the origin is well supported.

For Collectors

• Smaller stones are the entry point: 0.50–0.99 carat Zimmi stones are the most available
• Color comes first: pure yellow or orangy-yellow at the highest saturation
• Grade matters: the Zimmi premium is best supported for Fancy Vivid or exceptional Fancy Intense stones
• Clarity in context: intensely colored stones are judged more on color than clarity, but eye-clean remains the standard
• Keep the paperwork: retain all documentation supporting the Zimmi attribution

For Long-Term Holders

• Size: stones above one carat are the most sought after
• Color: Fancy Vivid is the grade most associated with the Zimmi premium
• Documentation: a GIA report plus written provenance
• Time horizon: colored diamonds are best considered long-term holdings, and prices can fall as well as rise

How much to allocate to any asset is a personal decision best made with your own financial advisors.

Realistic Expectations

Zimmi diamonds are not readily available like other yellow diamonds. Acquiring an exceptional stone may require patience, some flexibility on specifications, a willingness to act quickly when the right stone appears, and a higher acquisition cost that reflects the rarity premium.

Our Zimmi Standards

Our five-generation expertise and global network give us access to exceptional Zimmi yellow diamonds when they become available:

• ✓ Type Verified: laboratory confirmation of isolated (Type Ib) nitrogen
• ✓ Fancy Vivid Grade: Fancy Vivid preferred, with occasional exceptional Fancy Intense
• ✓ Exceptional Saturation: color consistent with true Zimmi quality
• ✓ Documented Provenance: written documentation of Sierra Leone origin
• ✓ Expert Evaluation: personal assessment by Brian Gavin
• ✓ Performance Cut: proportions that maximize color return
• ✓ Responsible Sourcing: Kimberley Process certified, traceable supply

Acquisition Services

For clients seeking Zimmi yellow diamonds, our VIP advisors provide:

• Active Sourcing: relationships with Sierra Leone sources and ongoing monitoring of availability
• Early Access: first viewing opportunities for exceptional stones
• Expert Authentication: careful verification of origin and quality
• Market Context: updates on availability and pricing
• Patient Acquisition: we wait for the right stone rather than compromise on quality

Current Availability

Given the rarity of Zimmi diamonds, availability fluctuates. We encourage interested clients to:

1. Contact us to discuss your interests and criteria
2. Join our VIP notification list for Zimmi acquisitions
3. Allow time — exceptional stones are worth waiting for
4. Be prepared to act quickly when opportunities arise

What the Supply Picture Suggests

No one can forecast prices, and we do not try. What can be observed is the supply side, and it is unusually well defined for Zimmi:

• A small alluvial field that has been worked for decades
• No industrial-scale expansion that would meaningfully change output
• Production governed by Sierra Leone mining regulation
• Growing recognition of origin as a value factor, as with Kashmir sapphire, Burmese ruby and Golconda diamonds

A finite source and a recognized name are the conditions under which origin premiums have historically held. Whether that continues is not something we or anyone else can promise — but it is why we buy Zimmi when we can get it.

Expert Guidance Available

Whether you are a first-time colored diamond buyer or an experienced collector, we can walk you through Zimmi diamonds, current availability and what to look for.

Contact us to:

• Discuss current Zimmi availability
• Join our VIP notification list for new acquisitions
• Review authentication and provenance standards
• Schedule a consultation with Brian Gavin

• GIA — Diamond Research Gives Clues to the Formation of the Continents (Zimmi age; Type Ib rarity)
• Smit, Shirey & Wang (2016), Precambrian Research; Smit et al. (2018), Mineralogy and Petrology (Zimmi Ib-IaA diamonds)
• Fancy Color Research Foundation — Why Are Zimmi Vivid Yellow Diamonds So Special?
• Natural Diamond Council — Diamond Report Series: Fancy Colour Diamonds (sources; FCRF index figures)
• GIA — Spring 2016 auction season; Rapaport and SCMP reports on the October 2024 Christie's Hong Kong sale

Expert Guidance at Every Stage

• VIP Advisor Services — Sourcing, collection guidance and jewelry design
• Authentication & Appraisal — Verification, valuation, Type classification testing
• Yellow Diamonds Guide — Complete yellow diamond education including market context
• Diamond Types: IIa, IIb, Ia & Ib — Understanding Type Ib science
• Diamond Care & Maintenance — Cleaning, storage, insurance, and inspections

All consultations are complimentary and without obligation.

All information provided is for educational purposes. RARE.DIAMONDS does not provide investment advice. Auction estimates are not sale prices, and past performance does not guarantee future results.