Understanding gems
Gem cuts explained
A cut is a set of compromises. Every shape trades brilliance against weight, colour against sparkle, or safety against drama — and the cutter decides which to keep before the first facet is placed.
Pick a cut to see what it does and why it exists. On Gems Guild, every listing states its shape, so you can go straight from a cut you like to the stones that carry it.
Gem
Variety
Drag to turn the stone · scroll to zoom
Blue Sapphire · oval
An illustration of the shape only. Colours on screen never match a stone in the hand, and no diagram can show how a real gem returns light.
Brilliant-style
Oval
An elongated brilliant — triangular and kite-shaped facets radiating from the centre, in a rounded outline.
Why a cutter chooses it
It keeps far more weight from the original rough than a round does. When the rough is worth thousands per carat, that saved weight is the difference between a stone worth cutting and one that isn't.
What it costs you
Stretch the outline too far and a dark band can appear across the centre — the 'bow-tie', caused by the pavilion shadowing itself.
In Ceylon
The default cut for Ceylon sapphire. Most rough comes out of the ground in shapes that suit an oval, so the cutter follows the stone rather than fighting it.
Why cut matters
Where the light goes
A gem has no light of its own. Everything you see is light that went in through the top, bounced around inside, and came back out — and whether it comes back at all is decided by the angle of the pavilion.
Drag the slider and watch the ray. There is a hard threshold at 34.4° for sapphire: below it the stone stops reflecting and simply leaks. This diagram traces the ray properly rather than illustrating it, so the moment it fails is calculated, not drawn.
The pavilion is steep enough that both facets reflect the ray completely, and it comes back up through the crown to the viewer. This is the whole job of a cut: catch light, bounce it inside the stone, and send it back out of the top. Fine Ceylon sapphire is usually cut around 40 to 43 degrees.
Angle of incidence at the pavilion: 41° · critical angle for sapphire: 34.4° · above it, so the facet reflects.
Weight is not size
What does a carat actually look like?
A carat is a fifth of a gram. It says nothing about how big a stone looks — that depends on how heavy the material is for its size, and gem species differ enormously.
These figures are exact, not illustrative: the trade’s weight formula run backwards. A one-carat round sapphire computes to 6.1 mm, which is what it measures in the hand.
Zircon
7.1 × 5.0 mm
SG 4.65
Sapphire
7.5 × 5.2 mm
SG 4.00
Spinel
7.8 × 5.4 mm
SG 3.60
Emerald
8.5 × 5.9 mm
SG 2.72
10 mm at this diagram’s scale — drawn to true proportion against each other, not life size on your screen.
All four stones weigh the same. The emerald is 20% wider than the zircon, because zircon is nearly twice as dense — the same weight of it takes up far less room.
This is why comparing stones by carat alone misleads you. A one-carat sapphire is noticeably smaller than a one-carat emerald, and neither is a better buy for it. When size on the finger is what you care about, ask the seller for millimetres — every listing here states them.