Unglazed porcelain plates rated to hardness 7, sized for the field notebook pocket. The single most decisive dollar-per-answer tool in identification.
Details
Unglazed porcelain, hardness about 7, which is exactly the point: anything softer leaves its powder behind, and the powder's color is more honest than the specimen's. Wet-erase with a thumb and water. The third plate is for the member who borrowed the first two.