Composition is the first-order control on hardness — in every burial group.

Over 1,500 rebound-hardness^{1Rebound (Leeb) hardness, HLD — a non-destructive surface-hardness measurement made by firing a small impact body at the rock and measuring the rebound velocity. Reported as a unit-less Leeb number (HLD); higher = harder. Quick, repeatable, and sensitive to composition and porosity in mudrocks.} measurements across four wells and 80 ft of core reveal clean silica-driven trends inside each diagenetic phase^{2Silica diagenesis — the stepwise transformation of biogenic opaline silica through burial: opal-A (amorphous) → opal-CT (cristobalite-tridymite) → quartz. Each step rearranges the silica fabric and changes porosity, density, and hardness. The mixed A-CT group is the transition zone between opal-A and opal-CT.}, plus a porosity story that inverts the textbook for siliceous mudrocks of the Monterey Formation.^{3The Monterey Formation is a uniquely siliceous Miocene marine unit of California, remarkable for its exceptional biogenic silica content, complex multi-stage diagenetic transformation (opal-A → opal-CT → microcrystalline quartz), and its dual role as California's dominant petroleum source rock and a major fractured-siliceous reservoir. Since Bramlette (1946): The Miocene Monterey Formation of California revisited — Behl, R. J. (1999).}