Detailed Answer

How to Evaluate Lab-Grown Old Mine Cut Diamonds: What to Look for in Elongated Cushion Shapes

A lab-grown old mine cut (OMC) elongated cushion diamond combines the antique, high-crown faceting architecture of 18th- and 19th-century old mine cuts with a rectangular length-to-width ratio greater than 1.20, producing a finger-flattering silhouette that reads larger than its carat weight suggests. Unlike modern brilliant cuts optimized by computer modeling, the OMC style is intentionally imprecise — its charm lies in slightly irregular facets, a small table, a high crown, and a large culet that scatter light in a warm, romantic way rather than the sharp scintillation of a round brilliant.

A real-world example illustrates the category well: a 3.61 carat lab OMC elongated cushion posted to r/LabDiamonds carries a 1.3 length-to-width ratio, D colour, VVS2 clarity, Excellent cut grade, 55% table, 64% depth, and IGI certification — priced at $3,700 CAD excluding the custom setting. That combination of specs gives a useful anchor for understanding what "strong on paper" actually means for this cut family.

Quick-Reference Comparison: OMC Elongated Cushion Spec Benchmarks

Parameter	Too Low / Avoid	Sweet Spot	Too High / Caution
Length-to-Width Ratio	Below 1.20 (reads square)	1.25 – 1.35	Above 1.45 (can look narrow)
Table %	Below 50% (very dark face-up)	53% – 58%	Above 62% (loses OMC character)
Depth %	Below 58% (shallow, leaky)	60% – 66%	Above 68% (face-up smaller, heavy)
Crown Angle	Below 30° (flat, modern look)	34° – 40°	Above 44° (very dark)
Culet	None (modern)	Small to Medium	Large (visible dot face-up)
Clarity	Below SI1 (inclusions visible)	VS2 – VVS2	FL/IF (premium rarely justified)
Colour	Below J (warmth visible in D/E stones)	D – G	— (personal preference above G)

The 3.61 ct example sits squarely in the sweet spot on table (55%) and ratio (1.3), while its depth of 64% is at the upper end of ideal — acceptable, but worth verifying that the stone does not look noticeably smaller face-up than its carat weight implies.

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What exactly is an old mine cut diamond?

Old mine cut refers to a diamond cutting style that predates modern mechanized bruting, characterized by a cushion-shaped outline, high crown, small table facet, large culet, and 58 facets arranged in a pattern that prioritizes carat retention from the rough rather than optical precision. The style dominated from roughly the 1700s through the early 1900s, when the round brilliant gradually replaced it.

Brilliant Earth's old mine cut guide describes the cut as having "a small table, large culet, high crown, and short lower half facets," which together produce a softer, warmer light return compared to modern cuts. That optical signature — often called "candlelight sparkle" — is the primary reason buyers seek OMC stones for antique-inspired engagement rings.

When the OMC outline is intentionally elongated during the cutting process, the result is an elongated cushion variant. This is not a standardized grading term; different cutters and vendors may label the same shape as "elongated cushion," "OMC elongated cushion," or simply "antique cushion." The elongation is achieved by adjusting the ratio of the stone's length to its width beyond the 1.00–1.05 range typical of a square cushion.

For lab-grown diamonds, the OMC elongated cushion is cut from CVD or HPHT rough using modern laser technology, but the facet geometry deliberately mimics the antique pattern. This means you get the aesthetic of an 18th-century stone with the consistency of modern production — and at a fraction of the cost of a natural OMC of equivalent size.

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How does the length-to-width ratio affect the look on the hand?

The length-to-width (L/W) ratio is the single most important shape parameter for an elongated cushion. It is defined as the stone's measured length divided by its width, expressed as a decimal (e.g., 1.30 means the stone is 30% longer than it is wide).

For a size 8 finger — which is wider than average — a 1.3 ratio on a 3.61 ct stone means the diamond spans roughly half the width of the finger, as noted by community members reviewing the r/LabDiamonds post. One commenter described it as looking "too large" and "disproportionate" for everyday wear, while acknowledging it has significant "wow factor." This tension is genuine in large elongated cushions: the same visual impact that makes a stone striking can also make it feel impractical for daily use.

Practical ratio guidance by finger size:

• Size 4–5 (narrow fingers): A 1.30–1.40 ratio elongates the finger beautifully without overwhelming it. Even a 2 ct stone at 1.35 will appear substantial.
• Size 6–7 (average fingers): 1.25–1.35 is the most versatile range. The stone elongates without looking disproportionate.
• Size 8+ (wider fingers): 1.30–1.40 still works, but carat weight becomes a stronger visual factor. A 3.61 ct at 1.30 will dominate the finger — which may be exactly what the wearer wants.

Ratios below 1.20 read as square cushions and lose the elongating effect entirely. Ratios above 1.40 can start to look narrow or "stretched," especially in the OMC style where the rounded corners already soften the outline. The r/LabDiamonds community consistently treats 1.25–1.35 as the sweet spot for OMC elongated cushions, and the 3.61 ct example at 1.3 sits precisely in that range.

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What table and depth percentages should you look for?

Table percentage is the width of the table facet expressed as a percentage of the stone's average girdle diameter. Depth percentage is the total height of the stone from table to culet expressed as a percentage of the average girdle diameter.

For modern brilliant cuts, a large table (around 57–62%) maximizes light return through the crown. For OMC stones, the opposite logic applies: a smaller table (53–58%) is part of what gives the cut its antique character, directing more light through the crown facets rather than straight down through the table. A table above 62% on an OMC begins to look like a modern cushion brilliant rather than a genuine antique-style cut.

The 55% table on the 3.61 ct example is textbook for this style — small enough to preserve the chunky, high-crown OMC look, large enough to avoid the very dark face-up appearance that can result from tables below 50%.

Depth is more nuanced. A depth of 60–66% is generally ideal for elongated cushions. Below 58%, the stone risks being "shallow" — light leaks out the bottom and the stone can look glassy or lifeless. Above 68%, the stone carries significant weight in the pavilion that does not translate to face-up size, meaning you pay for carats you cannot see. The 64% depth on the featured stone is at the upper end of the ideal range. It is worth asking the jeweller for the stone's actual millimeter dimensions (length × width × depth) to calculate the face-up area and compare it to other stones of similar carat weight.

For reference, a well-cut 3.61 ct elongated cushion with a 1.3 ratio should measure approximately 11.5 × 8.8 mm face-up. If the dimensions are significantly smaller, the depth is eating into visible size.

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How important is the cut grade and certification for OMC lab diamonds?

IGI (International Gemological Institute) is the dominant grading laboratory for lab-grown diamonds, and its certification is the industry standard for this category. An IGI report for a lab-grown OMC elongated cushion will include colour grade, clarity grade, cut grade, and the key proportions (table %, depth %, crown angle, pavilion angle, culet size, girdle thickness).

Cut grade for fancy shapes like elongated cushions is more subjective than for round brilliants, where GIA's Excellent grade has a well-defined parameter range. For OMC stones specifically, "Excellent" cut from IGI indicates that the proportions fall within acceptable ranges, but it does not guarantee the stone will be lively or beautiful in person. The faceting pattern of an OMC is intentionally irregular, so two stones with identical IGI grades can look very different face-up.

Brilliant Earth notes that old mine cut diamonds are "not held to the same strict cutting standards as modern diamonds," which means the cut grade is a starting point for evaluation, not a conclusion. The r/LabDiamonds community offers consistent perspective: "Specs are strong on paper, and the chunky OMC faceting gives it a nice antique look. Biggest thing is checking how lively it looks outside jewelry lighting."

This is the most important practical advice for buying an OMC lab diamond: always evaluate the stone in natural daylight or mixed lighting, not just under the bright spot lighting used in jewelry stores and product photography. Jewelry store lighting is designed to maximize scintillation in any stone; it flatters even mediocre cuts. Natural light reveals whether the stone has genuine life or goes dark and flat.

If you are buying remotely, request a video taken outdoors in overcast daylight. The Reddit video of the 3.61 ct stone is a good example of what to ask for — even a short clip in natural light tells you far more than a studio photograph.

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What colour and clarity grades make sense for an OMC elongated cushion?

Colour grading for lab-grown diamonds follows the same D–Z scale used for natural diamonds, but lab diamonds tend to show colour saturation differently from their natural counterparts — a topic explored in depth in our article on why lab-grown diamonds show different color saturation than natural diamonds.

For OMC elongated cushions specifically, colour choice interacts with the cut's optical character. The high crown and chunky facets of an OMC tend to concentrate colour in the body of the stone more than a shallow modern brilliant does. This means a J or K colour OMC may show more warmth than a J or K round brilliant of the same grade — which can be a feature (warm, antique look) or a drawback (visible yellow tint), depending on preference.

The D colour on the 3.61 ct featured stone is the highest possible grade — colourless. In a lab-grown diamond at this price point ($3,700 CAD for 3.61 ct), D colour is achievable without the extreme premium it commands in natural diamonds. If budget is a consideration, E or F colour will be visually indistinguishable from D in most settings and lighting conditions, and the savings can be redirected to a better setting or a higher depth-to-size ratio stone.

For clarity, VVS2 is excellent — inclusions are very, very slightly included and invisible to the naked eye. For an OMC elongated cushion, VS2 is generally the lowest clarity grade worth considering, as the large facets and open face-up appearance of the OMC style can make inclusions more visible than in a tightly faceted modern brilliant. SI1 stones in this style should be evaluated carefully with loupe inspection before purchase.

The combination of D/VVS2 on the featured stone is genuinely strong. Whether that premium over, say, F/VS1 is worth the cost depends on the buyer's priorities — but for a custom engagement ring intended to be worn daily, the peace of mind of top-tier specs has real value.

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Is $3,700 CAD a fair price for a 3.61 ct lab OMC elongated cushion?

Pricing for lab-grown diamonds has shifted dramatically since 2020, with prices falling as production capacity has expanded. As of mid-2026, a 3.61 ct lab-grown diamond in D colour, VVS2 clarity, and Excellent cut in a round brilliant would typically retail for $4,000–$6,000 CAD depending on the vendor. The OMC elongated cushion style commands a modest premium over standard cushion brilliants due to the specialized cutting required to achieve the antique facet pattern, but it generally trades at a slight discount to round brilliants of the same carat weight and grade.

At $3,700 CAD for the stone alone (excluding the custom setting), the 3.61 ct D/VVS2 OMC elongated cushion in the r/LabDiamonds post appears to be reasonably priced for the Canadian market. The custom setting will add cost — a simple solitaire in 14k gold might add $500–$1,000 CAD, while a more elaborate vintage-inspired setting with side stones could add $2,000–$3,000 CAD or more.

For comparison, a natural old mine cut diamond of 3.61 ct in D/VVS2 would cost $40,000–$80,000+ CAD, making the lab-grown version roughly 95% less expensive for an equivalent visual result. This is the core value of lab-grown OMC diamonds: the antique aesthetic at a fraction of the natural stone price.

When evaluating price, always confirm whether the quoted price includes the IGI certificate. A legitimate IGI certificate for a stone of this size should be included as standard; if the jeweller is charging separately for certification, that is a yellow flag worth investigating.

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How does the OMC elongated cushion compare to other antique-inspired lab diamond cuts?

The OMC elongated cushion sits within a broader family of antique-inspired cuts that are growing in popularity for lab-grown engagement rings. Understanding how it compares to alternatives helps clarify whether it is the right choice for a given buyer.

Old European Cut (OEC): The OEC is a round antique cut with the same high crown, small table, and large culet as the OMC, but in a circular rather than cushion outline. It is the predecessor to the modern round brilliant. OEC lab diamonds offer the same warm, romantic light return as OMC stones but in a round silhouette — better for buyers who want antique character without the elongated look. Our article on rare diamond cuts including old mine cut covers this comparison in more detail.

Modern Cushion Brilliant: The modern cushion brilliant uses a cushion outline but with a modern faceting pattern optimized for maximum light return. It lacks the chunky, high-crown character of the OMC and produces sharp, bright scintillation rather than the soft, diffuse glow of antique cuts. Buyers who want maximum sparkle in a cushion shape should consider the modern cushion brilliant; those seeking the antique aesthetic should choose the OMC.

Elongated Oval: The oval is a popular elongated alternative with a different facet pattern (modified brilliant) that produces a bowtie effect in the center when cut poorly. The OMC elongated cushion avoids the bowtie issue entirely due to its facet architecture. For buyers choosing between the two, the oval reads as more modern; the OMC elongated cushion reads as more antique and romantic. Our lab diamond shapes comparison guide covers oval vs. cushion in detail.

Radiant Cut: The radiant is a rectangular or square cut with trimmed corners and a brilliant facet pattern. It produces very high scintillation but lacks the antique character of the OMC. A 9 carat radiant discussed on r/LabDiamonds illustrated a common radiant issue: the cut style can make large stones look smaller than expected face-up due to its depth requirements.

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What should you check before finalizing a purchase?

Before committing to a lab-grown OMC elongated cushion, run through this evaluation sequence:

1. Verify the IGI certificate number independently. Every legitimate IGI-certified stone has a report number that can be verified on the IGI website (igi.org). Enter the number and confirm the specs match what the jeweller has quoted. Discrepancies in colour, clarity, or proportions are immediate red flags.

2. Request the actual millimeter dimensions. The IGI report will list length × width × depth in millimeters. Calculate the face-up area (length × width) and compare it to other stones of similar carat weight. A 3.61 ct elongated cushion with a 1.3 ratio should measure approximately 11.5 × 8.8 mm. If it measures 10.8 × 8.3 mm, the depth is eating into visible size and you are paying for hidden carats.

3. Evaluate in natural light. Jewelry store lighting flatters every stone. Ask to take the stone to a window or outside. An OMC that goes dark and flat in natural light is a poor performer regardless of its paper specs.

4. Check the culet. A large culet — the flat facet at the bottom point of the stone — is characteristic of antique cuts and is part of the OMC's charm. However, an excessively large culet can appear as a visible dark circle when viewed face-up. Ask the jeweller to show you the stone face-up under magnification. A small to medium culet is ideal; a large culet is acceptable if the visual effect appeals to you.

5. Assess the girdle thickness. A very thin girdle increases the risk of chipping, especially on the corners of a cushion shape. A medium to slightly thick girdle is preferable for everyday wear.

6. Consider the setting design early. OMC elongated cushions have a distinctive outline that requires a setting designed to accommodate their proportions. A standard cushion setting may not fit correctly. Custom settings, bezel settings, and four-prong solitaires designed for elongated cushions are all good options. The prong placement should protect the corners without covering too much of the stone's face-up area.

7. Think about lifestyle fit. The r/LabDiamonds community raised a valid point about the 3.61 ct stone: at half the width of a size 8 finger, it is a statement piece. For someone who works with their hands, exercises frequently, or prefers understated jewelry, a smaller stone or a bezel setting that protects the stone may be more practical. For someone who loves maximalist jewelry and treats their ring as a statement accessory, the size is a feature, not a drawback.

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How does the OMC elongated cushion perform as a lab-grown diamond specifically?

Lab-grown diamonds are chemically, physically, and optically identical to natural diamonds — they are defined as diamonds grown in a controlled laboratory environment using either Chemical Vapor Deposition (CVD) or High Pressure High Temperature (HPHT) processes, producing the same crystal structure as mined stones. The OMC faceting pattern performs identically in lab-grown and natural diamonds because the optical properties are the same.

Where lab-grown OMC diamonds differ from natural OMC diamonds is in availability and consistency. Natural OMC diamonds are antique stones — they were cut 100–300 years ago and exist in finite supply. Each one is unique, with its own idiosyncratic proportions. Lab-grown OMC elongated cushions are cut to order from modern rough, meaning you can specify a ratio, approximate dimensions, and quality parameters and have a stone cut to those specs. This is a significant advantage for custom ring projects.

The price differential is substantial. A natural 3.61 ct D/VVS2 OMC would cost $40,000–$80,000+ CAD; the lab-grown equivalent is available for under $4,000 CAD. For buyers who prioritize the visual aesthetic of the OMC style over provenance or investment value, the lab-grown option is objectively superior value.

For buyers considering lab-grown diamonds more broadly, our complete buying guide to lab-grown diamond solitaire rings and our roundup of the best lab-grown diamond engagement rings provide additional context on vendors, pricing, and setting options.

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What are the most common mistakes buyers make with OMC elongated cushions?

The most frequent errors fall into three categories:

Over-relying on paper specs. An IGI Excellent cut grade on an OMC elongated cushion does not guarantee a lively, beautiful stone. The OMC style has inherently more variation than modern brilliant cuts, and two Excellent-graded stones can look dramatically different in person. Always prioritize in-person or video evaluation over spec sheets.

Ignoring the depth-to-size relationship. A high depth percentage (above 66%) means the stone carries significant weight below the girdle that is invisible face-up. Buyers who focus only on carat weight without checking millimeter dimensions may end up with a stone that looks smaller than expected. The r/LabDiamonds post about a 9 carat radiant is a cautionary example: even at 9 carats, a deep stone can look "smaller than expected."

Choosing a ratio without trying it on. The L/W ratio looks like a number on a spec sheet but translates to a very specific visual on the hand. If possible, try on stones at different ratios — 1.25, 1.30, 1.35 — before committing. Many jewellers who specialize in antique-style lab diamonds will have sample stones or can source comparison stones for a try-on appointment. The r/EngagementRings community consistently recommends trying multiple shapes and ratios in person before deciding.

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The lab-grown OMC elongated cushion is one of the most compelling options in the current engagement ring market: it combines genuine antique aesthetic character with the accessibility and consistency of modern lab-grown production. The 3.61 ct D/VVS2 example at $3,700 CAD demonstrates that stones with strong paper specs are available at prices that would have been unthinkable five years ago. The evaluation framework — ratio, table, depth, certification verification, and live-light performance — applies to any stone in this category, regardless of carat weight or budget.