For leapfrogging (also called "re-referencing" or "monument spheres"), the requirements are different from probe tips — you need large, stable, highly spherical reference spheres that can be reliably re-acquired from multiple scanner positions.

Key Requirements for Leapfrog Spheres

• Low sphericity — to reduce registration errors
• Larger diameter — typically 1" (25 mm) to 4" for good angular coverage and scan acquisition
• Stable mounting — The spheres must not move during leapfrog moves.
• Material that scans well — good reflectivity or diffuse surface for the scanner type you're using.  (Not high polish.)

Best Materials

Tooling Steel (hardened) is the most common for permanent or semi-permanent monuments. Very stable, holds geometry well, and machines to excellent sphericity. It can rust if not coated or stored properly.

Stainless Steel is the practical choice for shop-floor use — corrosion-resistant, stable, and widely available in precision grades. Slightly less hard than tooling steel, but excellent for leapfrog monuments.

Ceramic (Alumina/Zirconia) offers outstanding dimensional stability and is chemically inert. More expensive but ideal if thermal stability is critical, since ceramics have very low, consistent CTEs.

CTE describes how much a material expands or contracts per degree of temperature change. Usually expressed as µm/m/°C (or in/in/°F).

Quick reference:
Steel — ~12 µm/m/°C
Stainless Steel — ~17 µm/m/°C
Alumina ceramic — ~8 µm/m/°C
Invar — ~1.2 µm/m/°C (extremely stable, used in high-precision metrology fixtures)
Carbon Fiber — ~1-3 µm/m/°C depending on layup

For a 1.5" steel sphere with a 5°F shop temperature swing, you're looking at roughly 0.00001" dimensional change — usually negligible. But for large-volume leapfrogging across a big shop with significant temp swings, ceramic or Invar monuments become worthwhile.
For metrology, it matters because a sphere that expands or contracts between your first and second scanner setup will introduce registration errors in your leapfrog. Materials with low, consistent CTE maintain their geometry better during temperature fluctuations on the shop floor.

Carbon Fiber / Composite Mounted Spheres — the sphere itself is usually steel or ceramic, but the mount matters. Lightweight CF mounts reduce monument shift from vibration.

Practical Considerations for VTube-LASER

• For arm leapfrogging, 25 mm (or greater) spheres
• Ensure sphericity is certified to < 25 microns (0.000984 inches)
• Nesting consistency matters as much as sphere quality when using tactile — a precise sphere on a sloppy mount defeats the purpose. It does not matter as much for LASER scanning, because LASER is non-contact.  It is critical that the sphere not move during the leapfrog process.
• A matte or lightly bead-blasted finish always scans better than mirror polish.