A stemming plug is a small component with a compounding effect on a blast. Its job is to keep explosive energy confined in the hole long enough to fracture rock. When stemming ejects too early and that energy escapes, the consequences can affect a site’s profits, regulatory compliance, and even safety.
Varistem and RockLock are two widely used stemming solutions. We compare their pricing, hole-diameter range, explosive compatibility, mechanism, and verified results, providing blast engineers, site managers, and procurement teams with the information needed to make a clear choice that meets your site’s needs.
Varistem plug prices scale with hole diameter, ranging from approximately US$1 for smaller sizes up to around US$16.13 for the largest. This means you only pay for the size the hole actually requires, which helps keep the cost per hole predictable across blast patterns where hole diameters vary.
RockLock pricing scales with hole diameter and is not universally published. Most distributors require a quote. Where retail pricing is available, the 90mm ball works out to approximately US$7.43 per unit when bought by the box, while the larger 200mm balls are listed at US$17.77 per unit with a minimum purchase of 22 stemming balls.
Varistem has a clear price advantage. Its published diameter-based pricing is straightforward to budget at the planning stage. RockLock’s retail pricing, where available, starts considerably higher at around US$7.43 per unit for smaller sizes, and most distributors require a quote rather than offering transparent shelf pricing.
| Hole Diameter | Varistem | RockLock |
|---|---|---|
| 2.25″–2.75″ (45mm) | $1.00 to $1.58 | Quote required |
| ~3.5″ (90mm) | $2.00 | ~$7.43 (by the box) |
| 4″–5″ (102 to 127mm) | $2.78 to $3.73 | ~$7.43 to $10.84 per unit |
| 5½″–6½″ (125 to 137mm) | $5.83 to $6.08 | ~$10.84 to $11.73 per unit |
| 8″–8.5″ (203 to 216mm) | $12.15 | Quote required |
| 9″–9.875″ (200mm) | Up to ~US$16.13 | ~$17.77 per unit |
| 10″ / 235–251mm | $16.13 | Quote required |
Varistem is available for blasthole diameters from 1.75 inches (45mm) to 10 inches (251mm). Within that range, the full lineup covers 1.75″ to 10″, giving you a dedicated plug size for most standard production hole diameters. Each size is matched closely to a specific diameter window.
RockLock is available across a range of sizes, from the 45mm ball suited to holes of 2.25″–2.75″ to the 235mm ball designed for holes up to 10.75″. Confirmed sizes in between include 90mm through 150mm (covering up to 7.375″), and 200mm for 9″ to 9.875″ holes. No listed size covers the 7.375″ to 9″ range.
Both products cover the standard production range from 1.75″ upward. Where Varistem pulls ahead is its size increments. They are tighter and more clearly documented, making it easier to match a plug precisely to the drilled hole diameter. RockLock’s size windows tend to be broader, and the full-size list is harder to verify without going through a distributor.
Varistem works alongside the full range of commonly used commercial explosives, ANFO, emulsions, heavy ANFO, and water gels. Your explosive selection is driven by hole conditions and site practice as normal, with ANFO suited to dry holes and emulsion or water gel the go-to where water is present.
RockLock is an inert plastic sphere and presents no known compatibility constraints. It has documented use with ANFO and bulk explosives in open-pit and quarry applications.
Varistem has the edge simply because its compatibility is explicitly documented across a broader range. RockLock’s inert design means it’s unlikely to cause problems regardless of explosive type, but that compatibility isn’t formally stated.
Varistem is placed point-down into the blasthole, coming to rest directly on top of the explosive column. Stemming material is then loaded above it as normal. On detonation, the initial shockwave forces the plug upward into the stemming column, where its flexible, tear-resistant body wedges against the borehole wall and forms a temporary pressure seal. That seal holds long enough to redirect more explosive energy into the surrounding rock before the flame front destroys the plug.
RockLock is a hollow plastic sphere that is partially filled with stemming material before being dropped into the blasthole. Standard stemming is then loaded above it. On detonation, blast pressure forces the sphere upward, where its pliable body flexes against the borehole wall, creating a seal that absorbs and equalises pressure across the borehole.
Both plugs work on the same blast principle, the difference is in deployment. Varistem drops straight into the hole ready to use, with no preparation required on site. RockLock requires the operator to partially fill the sphere with stemming material before it goes in, adding a handling step per hole. On a large blast pattern, that time difference adds up.
Varistem publishes independently verified case study results across multiple site types. At one underground platinum mine, advance per blast improved from 2.46 m to 2.82 m — a gain of around 15% — with the associated case study estimating an annual net benefit of R1.19 billion linked to improved ore recovery.
Other documented outcomes include flyrock reductions of up to 80%, an 87% improvement in cast, crusher throughput gains of more than 14%, loading rate improvements of up to 8%, more than US$10 million per year in added value, and approximately US$1.45 million in savings from reduced waste blasts.
Advanced Blasting Technology states that RockLock improves aggregate fragmentation, enables explosives to be loaded higher into cap rock, and greatly reduces flyrock occurrences. The product has been in use across open-pit mining and quarry operations for more than a decade. They describe it as delivering substantial return on investment and significant bottom-line savings.
Both products have been used across mining and quarry operations. However, there is a significant difference in how performance is documented. Varistem publishes independently verified, quantified case study data. RockLock relies on general claims from the manufacturer and has no publicly available data to back the claims with numbers. If proof of performance matters to your decision-making process, Varistem has a clearer paper trail.
Choose Varistem when you want a straightforward, cost-effective stemming plug with a proven track record and transparent performance data. It covers blasthole diameters from 1.75″ to 10″, fits the full range of standard commercial explosives, and drops into the hole ready to use with no preparation required on site. Varistem is a strong fit for operations that value documented results, easy deployment, and predictable per-hole costs.
Choose RockLock when your operation uses decking as part of your blast design or needs to load explosives higher into the cap rock. Its design specifically supports loading into cap rock and delivers enhanced vibration control in decking applications, making it a practical fit for sites where those are primary concerns.
Varistem and RockLock are both proven stemming solutions, but they suit different priorities. Varistem is the stronger all-around choice with independently verified results, transparent pricing, a full size range, and no on-site preparation required. RockLock is worth considering when decking or cap rock loading is central to your blast design.
Contact us to find the right size for your holes and get a quote for Varistem stemming plugs.
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