Crystal Slabs, Slices and Freeforms: Cut Angle, Finish and Price

Short answer: a slab or slice is a flat section sawn through a nodule, geode or block and polished on one or both faces; a freeform is a chunk ground and polished to follow its own outline instead of a set geometric shape. Both exist to display the inside of a stone, so the angle of the cut matters more than anything else: across agate banding for the pattern, parallel to labradorite's internal layers for the flash. Judge flatness, polish to the edge, color honesty and how the piece stands.

Slab, slice, end cut or freeform: which is which?

The words overlap in listings, but they describe different cuts.

The Minerals.net glossary defines a slab simply as a cross-section slice cut out of solid rock or mineral, adding that agate slabs are often polished. A freeform has no single definition, but in practice it means the lapidary chose the stone's own contour over a sphere, tower or heart, which usually wastes less material and lets the best face stay large.

How does a nodule become a slab?

Reading the stone before the saw

A slab only shows what the saw plane passes through, so the cutter studies the rough first. Agate nodules are the clearest example of why. Geology.com describes a polished agate slab cut from a nodule whose bottom was first infilled by horizontal layering of silica, then concentric infilling, then another episode of horizontal filling. Saw that nodule in the wrong plane and the story collapses into a few stripes; saw it through the center, perpendicular to the layers, and every stage shows. The agate banding guide covers how those layers form.

Choosing the angle

For a patterned stone, the cut goes across the structure. For a stone with an optical effect, the cut follows it. Iris agate is the extreme case. A 1952 study by Francis T. Jones in American Mineralogist found that the slice shows the brightest colors if it is thin (1 mm or less) and cut so the fine layers sit at right angles to the polished surface; a slice cut too thick may show only pastel colors. Jones measured grating structures ranging from about fifteen thousand to as few as four hundred lines per inch. Geology.com's iris agate article gives the same rule in plain terms: candidates must be sliced so that the sawn surface is perpendicular to the banding, and the illumination must come from behind the stone.

Labradorite works the other way round. Its flash is light reflecting from internal twinning surfaces, and Geology.com explains that if the stone is cut at any other angle than parallel to those layers, the flash looks inclined and off-center. A good labradorite freeform has its broadest polished face set to that plane, which is why two freeforms of similar size can differ sharply in price.

Grinding, sanding and polish

After sawing, saw marks are ground out, the face is taken through finer abrasives, and the final polish is applied. Freeforms add a shaping stage: the lapidary grinds off weak, fractured or dull material and rounds the outline so the piece stands or lies stably. On a slab, the edge is often left natural, as with a rose quartz slice whose polished face sits inside a rough rind. That contrast is part of the appeal, but it should be a choice, not a shortcut on the face.

Which stones reward a flat cut?

The International Gem Society notes that rose quartz is nearly always cloudy or translucent, rarely transparent, which is why slices suit it: thin sections let light through a stone that looks opaque in bulk. For rhodonite, the same society names Russia's Sverdlovsk region as the source of very fine and rich, massive, pink material, the kind that makes the best freeforms and slabs. Labradorite, at a hardness of about 6, scratches more readily than agate and jasper, so polished labradorite faces show handling wear sooner.

Labradorite also comes in architectural sizes. Geology.com notes that some deposits of anorthosite, the rock that can be mostly labradorite, are quarried and cut into slabs used for small sculptures, countertops and tiles. A decorative labradorite slab and a countertop offcut are the same material at different grades.

Why are so many agate slices dyed?

Because agate is porous and takes color easily, and because natural agate is often gray or tan. Geology.com shows a matched pair of agate bookends where the blue color was produced with dye, and it describes agate generally as a porous material that readily accepts dye. Thin slices are the form most often dyed, since dye penetrates a few millimeters of stone more evenly than a thick block. Neon pink, teal and electric blue slices are colored by people. How the dyes are applied and how to check a slice are covered in the dyed agate guide, so this page does not repeat them; the short version is to treat any vivid matched set as treated until shown otherwise.

What separates a well-finished slab or freeform?

• Flatness. Tilt a slab against a window reflection. A flat face shows one clean reflected edge; a wavy one bends it.
• Polish to the edge. The polish should run right to the rim. A dull band near the edge means the lapidary stopped early.
• No undercutting. Banded stones contain soft and hard layers. Poor sanding leaves the soft layers lower, visible as ridges under raking light.
• Fractures declared. Cracks show as bright lines when light comes from the side. Some are filled with resin; a fill looks glassy and slightly sunken.
• Stability. A freeform should stand on its own base or come with a stand that fits. Slabs over a few hundred grams need a stand with padded contact points.
• Light. Slices are meant for backlight. Look for a listing photo against a window or lamp, as iris agate in particular shows nothing in reflected light.

How do size, thickness and pattern affect price?

Price tracks four things. Pattern and flash come first: landscape agate scenes, according to Geology.com, are prized by collectors, and a full-face labradorite flash outprices a patchy one of the same weight. Size comes second, since large flawless sections are scarcer than small ones. Thickness matters for thin slices because a 2 mm slice that survives sawing and polishing without cracking took more skill than a thick slab. Finish quality is last but easy to see. Dyed slices sit at the bottom of the market because the color, not the stone, is doing the work.

Shopping slabs, slices and freeforms in stock

Ask for the dimensions in all three directions and a backlit photo for any slice. For a freeform, look for a view of the base and a side view to judge depth.

• Labradorite Crystal Freeform XL Polished Labradorescent: 203.2 x 167.64 x 83.82 mm, shaped to the stone's own contour, a size where face orientation is worth checking from several angles.
• Moroccan Agate Crystal Geode Slice Polished Display: a geode cross-section with concentric agate bands; compare its natural tones with the dye colors described above.
• Amethyst Laced Agate Crystal Slab Purple Banded Stone: a slab where agate banding and amethyst share one face, a good test of a cut made across the structure.
• Rose Quartz Crystal Polished Slice Natural Pink Stone: a polished face with the natural edge left on, showing the cloudy translucency typical of rose quartz.
• Pink Amethyst Polished Slab Sculpture on Metal Display Stand 3.11kg: an 8 x 1.25 x 16 inch slab from Argentina supplied on a metal stand, the right way to display a heavy upright slab.
• Flower Agate Freeform: an affordable freeform where the plume and flower patterns sit inside translucent chalcedony.

Frequently asked questions

What is the difference between a slab and a slice?

Mostly thickness. A slab is a thicker flat section that stands or lies as a solid piece; a slice is thin enough to pass light and is usually shown on a stand or in a window. Both are cut the same way.

Is a freeform natural or cut?

Cut. The outline follows the stone's natural shape, but the surfaces are ground and polished by a lapidary. Freeforms keep more of the original material than geometric shapes, which is why they suit stones with a large display face.

Why does my labradorite freeform only flash from one side?

The flash comes from internal twinning layers, which run in one direction through the stone. Only faces cut near parallel to those layers show full color face-up. Other faces show gray or flash at a steep angle.

How thin should an agate slice be?

For ordinary display, thin enough for light to pass, a few millimeters. For iris agate, a 1952 study found the brightest spectral colors in slices 1 mm or less, cut at right angles to the fine banding.

Can dyed agate slices fade?

It depends on the dye. Stability varies with the chemistry used, so a dyed slice is best kept out of a sunny window as a precaution. The dyed agate guide covers which colorants are used and how to tell dyed from natural color.

Sources

• Minerals.net, Glossary: Slab: https://minerals.net/glossary/slab
• Geology.com, Agate Gemstones: https://geology.com/gemstones/agate/
• Mineralogical Society of America, Iris Agate (F. T. Jones, American Mineralogist, 1952): http://www.minsocam.org/MSA/collectors_corner/arc/iris.htm
• Geology.com, Iris Agate: https://geology.com/stories/13/iris-agate/
• Geology.com, Labradorite: https://geology.com/gemstones/labradorite/
• International Gem Society, Quartz Value, Price, and Jewelry Information: https://www.gemsociety.org/article/quartz-jewelry-and-gemstone-information/
• International Gem Society, Rhodonite Value, Price, and Jewelry Information: https://www.gemsociety.org/article/rhodonite-jewelry-and-gemstone-information/