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Blue Diamond

Last updated: April 2026

Blue color in Type IIb diamonds is caused by trace boron. Natural Type IIb diamonds are extremely rare; most blue diamonds in commerce are color-treated or synthetic. All standard diamond ID properties apply (RI > 2.40, SR, hardness 10, adamantine luster); SW UV phosphorescence is the key field differentiator from other blue stones.

Blue Diamond has a specific gravity of 3.52 and a refractive index of 2.417, with a Mohs hardness of 10. It is singly refractive.

Physical & Optical Properties

RI Range2.417
SG Range3.50–3.53
SG Typical3.52
Hardness (Mohs)10
Crystal SystemCubic
Optic CharacterSR (Singly Refractive)
Dispersion0.044
Fluorescence LWInert
Fluorescence SWStrong orange
Chelsea FilterInert
PleochroismNone
ColorsBlue Violet
SpeciesDiamond
VarietyFancy Blue
Blue Violet

Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.

Through the refractometer

Blue Diamond no reading — over the limit 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORead the scale from bottom to top, then look for theliquid’s own faint line near the top. SEEThe field stays dark, with one faint line near 1.81 thatdoes not move when you change stones. MEANSOver the limit — this stone’s index is above the contactliquid’s own, so the liquid is the last thing the instrumentcan still see. That faint line is your proof the instrumentis working rather than dead. It is a RESULT: it eliminatesevery species below 1.81 in one observation. A field darkwith NO line means something else has gone wrong. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

Through the polariscope

Blue Diamond Dark throughout (SR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Blue Diamond stays dark through the whole rotation. That is singly refractive: there is no second ray to rotate the plane of vibration back into the analyser. Computed from optic character and transparency in gems.json. A polariscope separates singly refractive, doubly refractive and aggregate — it does not show the optic sign.

Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.

Specific gravity, and what it rules out

Blue Diamond SG 3.50–3.53 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 9 other species in the table overlap this range, so a reading here narrows the field to 10 rather than naming the stone. Combine it with a refractive index to separate what is left. Computed from the specific-gravity range in gems.json. Hydrostatic weighing — the bench method this site recommends; heavy liquids are toxic and regulated. Faint ticks are the other species in the table.

Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.

Through the dichroscope

Blue Diamond One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Blue Diamond shows no pleochroism, so both windows match at every orientation — turning the instrument changes nothing. That is a real negative result, and the reason it is worth knowing is below. Computed from pleochroism and pleochroic colours in gems.json. Window tints are derived mechanically from the colour words, not chosen; a neutral pair depicts no colour at all.

Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.

Brown DiamondDiamondPink DiamondYellow Diamond

Key Differentiators

Natural vs. Synthetic

Synthetic blue diamond is commercially available (HPHT (boron-doped), CVD (boron-doped)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.

Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for Blue Diamond.

Common Simulants

Commonly Confused With

Commonly confused with: Sapphire, Aquamarine, Spinel, Tanzanite, Iolite.

Treatments

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying a blue diamond? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Blue Diamond is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

Practice exams →