Identifying Rough Rock: What to Look For Before You Cut

The Art of Evaluating Rough Rock

Buying rough rock is an exciting gamble. A dull, dirt-covered boulder can hide spectacular colors and patterns, or it can be a crumbly, fractured mess that falls apart on your saw. Whether you are hunting for material at a lapidary tailgating event, a gem show, or buying online, learning how to evaluate rough rock is an essential skill that will save you time, money, and frustration.

Understanding Mohs Hardness

The most important factor in determining how a rock will cut and polish is its hardness, measured on the Mohs scale from 1 (talc) to 10 (diamond).

Most popular lapidary materials—agates, jaspers, petrified wood, and quartz crystals—hover around a 7 on the Mohs scale. This is the sweet spot for lapidary work. It is hard enough to take a brilliant, glass-like polish, but not so hard that it rapidly wears out your diamond wheels.

Stones significantly softer than a 5 (like fluorite, malachite, or calcite) require specialized, delicate techniques, as they scratch easily and often require different polishing compounds. When evaluating a piece of rough, a quick scratch test in an inconspicuous area with a steel nail (hardness of ~5.5) or a piece of quartz can help you estimate its hardness.

Windowing the Rough

It is nearly impossible to evaluate the interior of a rock when it is covered in a thick, opaque outer crust (the "rind"). Reputable sellers will often cut a small, flat section off one side of the rock—a process called "windowing."

If a rock has a window, spray it lightly with water or wipe it with a damp cloth. Water temporarily mimics the look of a polished surface, allowing you to see the true color, contrast, and pattern hidden inside. If you are at a show and the seller allows it, carrying a small spray bottle of water is a standard practice.

Checking for Fractures and Pits

The most beautiful pattern in the world is useless if the rock is shattered inside. Fractures are the enemy of the lapidary. As you run the rock through a slab saw, existing fractures will cause the slab to break apart.

To check for fractures, wet the stone and examine it closely under bright light. Look for thin, white lines or irregular, jagged lines cutting across the grain of the stone. Another technique, used primarily for clear materials like quartz or obsidian, is to "candle" the stone by holding a bright flashlight directly against it. The light will illuminate the interior, and any internal fractures will reflect the light brightly, making them obvious.

Also, look out for "vugs" or pits. These are small cavities in the rock that may be lined with drusy quartz or just filled with dirt. While a vug can look interesting in a specimen, if it lands in the middle of a cabochon you are trying to cut, it can ruin the piece by creating an un-polishable hole.

Agate vs. Jasper vs. Chalcedony

Knowing what family of rock you are looking at helps you predict how it will behave on the wheels.

• Agate: A translucent variety of microcrystalline quartz, characterized by distinct banding, moss-like inclusions, or plumes. Agate is very hard, cuts beautifully, and takes an exceptional polish with cerium oxide.
• Jasper: An opaque variety of microcrystalline quartz. It does not show light through it. Jasper gets its color from heavy mineral impurities (like iron, which makes it red). It generally cuts just as well as agate but may require slightly more time on the pre-polish wheels.
• Chalcedony: The broad term for all microcrystalline quartz, but in lapidary, it usually refers to pieces that are translucent but lack the distinct banding of agate (e.g., blue chalcedony or carnelian).

Estimating Yield and Value

When buying a large piece of rough, try to visualize how it will sit in your saw's vise. Irregular, knobby shapes are difficult to clamp securely and will result in a lot of wasted material (slabbing scrap). A blocky, relatively square piece of rough will yield a much higher percentage of usable slabs. Consider the price per pound versus the estimated usable yield when deciding if a piece of rough is a good deal.