Underground

Drop Frequency

Name	Level	Experience	Gold Value	Copper	Tin	Iron	Gold	Mithril	Addy	Runite	Clay	Stone	Sand	Silver	Coal	Slate	Stygian	Salt
Underground	40	1-50	1-100			5	25	20				5		15	5	15		10

Note that these numbers represent a frequency, that's why they sometimes add up to more than 100 in total.
To get percentage values you need to do (frequency / totalFrequency) * 100, i.e. Clay in Desert: (3 / 106) * 100 = 2.83%