Short answer
Two values used to describe rock get confused with each other: compressive strength (UCS, resistance to breaking) and abrasivity (how much it wears equipment). These are independent properties. A soft but abrasive sandstone accelerates tool consumption while offering little resistance to impact; a hard but low-abrasion limestone demands strong impact but does not tire tool life. The attachment decision has to look at both axes together, not just one.
Two different properties, two different outcomes
Compressive strength describes how much the rock resists breaking, and it sets the impact energy required. High-strength rock needs a breaker with higher operating pressure or a stronger drum cutter.
Abrasivity describes how much the hard minerals inside the rock, quartz in particular, wear surfaces. High abrasivity shortens the life of tools, chisels or picks regardless of the attachment impact power.
Because these axes are independent, four combinations arise: soft, low-abrasion (most breakers handle it easily), soft, high-abrasion (some sandstones, where consumable cost dominates), hard, low-abrasion (some limestones, where power is needed but consumables stay cheap), and hard, high-abrasion (quartz-rich granite, the most demanding combination of all).
How classification is done
Compressive strength is measured with a uniaxial compression test on a sample in the laboratory, and internationally recognised classification systems (such as ISRM) group rock into categories from "very weak" to "extremely strong".
Abrasivity uses standard tests such as the Cerchar abrasivity index; the quartz content of the rock affects this value directly.
These measurements need a geotechnical laboratory and vary widely by rock type, which is why we do not give a general threshold here. If your project has this data (a geotechnical report, a quarry licensing document), assessing the attachment decision alongside it is the sound approach.
What if there is no data?
On sites without a lab test, rock behaviour can also be read by observation: a rough, sharp-edged breaking surface indicates high strength; shiny, crystalline grains visible on the broken face (particularly white-grey quartz grains) can indicate high abrasivity. This observation is not a substitute for measurement, but it guides an initial decision.
What it means for the attachment decision
- High strength, low abrasivity: a powerful breaker is enough, tool consumption is not a concern. Standard tool type can continue.
- Low strength, high abrasivity: breaker power is not heavily challenged but the tool wears quickly. A dust channel tool and frequent tool checks add value here.
- High strength, high abrasivity: both a powerful attachment and durable consumables are needed. Hard rock tools pay for themselves fastest here.
- If a drum cutter is chosen, abrasivity directly sets pick life; in high abrasivity, pick consumption can become a larger cost line than tool consumption is for a breaker.
The same quarry, different behaviour
Even within a single quarry, rock properties can change layer by layer. A weathered layer near the surface may show low strength, while fresh rock deeper down shows higher strength and a different abrasivity.
So the attachment decision should not be made once for the whole quarry, but reviewed as layers change. A sudden rise or fall in tool consumption is the first sign a layer has changed.