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Mastery Tokens give 0.1% of the maximum mastery pool exp to mastery pool of the corresponding skill when they are claimed. Mastery Tokens are earned randomly while skilling in a noncombat skill. The drop chance for mastery tokens is 1 in [math]\frac{18\,500}{\text{Milestones unlocked for skill}}[/math]. Each thing you unlock by leveling the skill counts as a milestone e.g. smithing steel platelegs is unlocked at level 40 and counts as one milestone, additionally reaching level 99 in a skill counts as one milestone. Wearing Clue Chasers Insignia increases the drop chance by changing it to 1 in [math]\frac{18\,500 \times 0.9}{\text{Milestones unlocked for skill}}[/math].
Mastery Token Chance at Level 99