How to Use a Sliding Caliper for Gemstone & Watch Work
By Jewellery Tools UK — Hand Tools for Jewellers & Silversmiths | Published: 2026-10-29
Category: How-to Guides
Learn how to measure gemstones, beads, watch components, and ring inside diameters accurately with a brass sliding caliper. Step-by-step guide for jewellers and watchmakers.
Quick answer
Use a brass sliding caliper with notched jaws to measure gemstones, beads, and watch components accurately without scratching or magnetising delicate parts.
Why brass matters for gemstone and watch work
A brass caliper is non-magnetic, so it will not interfere with the delicate balance of a mechanical watch movement. Its softer metal also reduces the risk of scratching gemstone surfaces or polished metal parts, making it a safer choice than steel alternatives.
The notched jaw design is specifically shaped for inside-diameter readings — essential for measuring ring bezels, watch case openings, and the inner diameter of beads or spacers.
How to measure with a sliding caliper
- 1. Clean the item and zero the caliper: Wipe the gem, bead, or watch component with a soft cloth to remove dust or oil. Close the caliper jaws fully and check that the zero mark on the sliding scale aligns with the main scale. If not, gently adjust the slide until it does. A dirty or misaligned caliper gives false readings. Always zero before each session.
- 2. Measure outside dimensions (gem, bead, watch case): Place the item between the large flat jaws. Gently close the jaws until they just touch the surface — do not squeeze. Read the measurement at the edge of the sliding jaw's mark on the main scale. Overtightening can chip a gem or deform a soft metal bezel. Stop at first contact.
- 3. Measure inside dimensions (ring bezel, watch opening, bead hole): Insert the notched jaws into the opening. Open them until they contact the inner walls. Read the value on the scale. The notches are designed to give a stable contact point for round or irregular interiors. Tilt the caliper slightly to ensure both notches touch the inner walls evenly — an angled reading will be too small.
- 4. Measure depth (watch case, setting depth): Extend the depth rod (the thin bar that slides out from the end of the caliper) into the cavity. Rest the caliper body flat on the rim. Read the depth where the rod meets the main scale. Ensure the caliper body sits flush on the rim — any gap adds error to the depth reading.
- 5. Record and double-check: Write down the measurement immediately. Repeat the reading at a slightly different spot on the item to confirm consistency. For round items, take two measurements at 90 degrees apart. Gemstones and watch components can be slightly out of round. Averaging two readings gives a more reliable size.
Product facts
- Sliding Caliper — 4 Inch (100 mm) Brass, Notched Jaws for Gem, Bead & Watch Work: A brass sliding caliper with notched jaws for inside-diameter readings, dual metric and imperial scales, and a non-magnetic body safe around watch movements. You can measure ring and bezel inside diameters, watch components, gem and bead sizes without scratching or magnetising your work.
Sliding Caliper — 4 Inch (100 mm) Brass, Notched Jaws for Gem, Bead & Watch Work
