Metaquartzite and novaculite are very fine-grained and were used for lithic tools, and both are gray, tan, brown, and pink, making them sometimes hard to identify.  Because both rocks have a very fine texture, differentiating between them macroscopically, or even with a hand lens, is not always possible.  I used the electron microprobe to show the mineralogical differences in novaculite and metaquartzite with similar colors and textures.

Metaquartzite is a quartz-rich metamorphic rock.  The original quartz grains have been fused by heat and pressure, giving the rock a metamorphic micro-texture.  It often contains 10% or more of other minerals, including iron oxides, carbonates, and clay minerals.  Novaculite, on the other hand, is a micro- or cryptocrystalline sedimentary rock that usually is nearly pure silica -- carbonates, like calcite, have leached out of the material.  The backscattered-electron (BSE) images and X-ray element maps below easily show these mineralogical differences.

Metaquartzite and novaculite are sometimes hard to distinguish macroscopically, but X-ray microanalysis cannot be fooled by appearances -- their colors and textures can vary highly but not their mineralogy.