Gemology Fast Track / Unit 23 of 24 · Grading · 60 min

Evaluating and buying rough

Goal: Assess rough for species, colour, clarity, orientation and yield before you pay.

Rough inspection routine

1 · IDSG (works perfectly on rough), RI on a window, polariscope, habit, inclusions.
2 · ColourUnder daylight + incandescent; look through the stone, not at the skin. Wet it.
3 · Map the insideStrong torch (e.g. 'rough light') from behind: mark fractures, inclusions, zoning with a marker.
4 · Find the axisPleochroism and zoning decide the table direction.
5 · Plan the cutShape that fits the clean colour zone; estimate finished size and weight.
6 · Price backwardsFinished weight × finished price/ct − cutting cost − risk margin = max rough price.

Colour zoning and the table

colour zone in culet → colours the whole face

A small patch of colour placed low in the pavilion (near the culet) reflects through the entire face-up view. Sapphire cutters use this to make a pale stone with one blue spot face up blue.

Zoning parallel to the table shows as stripes: avoid.

Common rough by species: what to check

Sapphire/rubySilk (star or heating candidate), zoning, parting planes, geuda potential.
EmeraldFractures reaching surface, oil already in rough, colour depth. Expect low yield.
TourmalineClosed c-axis, colour zoning, liquid feathers (trichites) in reds.
SpinelOctahedral faces natural? Fingerprints, colour evenness.
Aquamarine/topazClean material expected; pay for size and colour. Topaz cleavage planes.
GarnetDarkness: thin cuts or hollow cabs for dark almandine.

Red flags when buying rough

Synthetic 'rough'

Broken boule pieces (curved surfaces, conchoidal everywhere), uniform colour, no skin.

Glass / slag

Bubbles, swirls, SG off, warm feel.

Treated rough

Glass-filled ruby rough, dyed quartz, oiled emerald rough sold 'as mined'.

Salted parcels

Good stones on top, poor below. Inspect every piece or price the worst.

Wet display

Rough shown in water hides fractures; dry it too.

Weight claims

Weigh yourself; ask for scale tolerance.

Members can use the Rough evaluator tool: enter rough weight and planned finished dimensions; it estimates finished weight (GIA Gem Society formulas), yield % and a maximum rough price from your own price inputs.

Practical
Take a rough stone. Weigh it, SG it, light it from behind and sketch the clean zones. Use the Rough Evaluator to plan the biggest oval and round that fit, and compare yields.
Unit quiz

0/6 answered

1. Best test to ID rough quickly without polishing:
2. A colour spot near the culet will…
3. Rough with curved surfaces and uniform colour may be…
4. Maximum rough price is set by…
5. Why dry rough shown in water?
6. For dark tourmaline rough, place the table…
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