Colorado is second only to California in the variety of minerals it has within its boundaries. The state has over thirty varieties of gems. There are many opportunities for mineral collecting, but a person must be careful when evaluating a collecting site. A given mineral locality may actually be a staked claim. Collecting from such a site without permission is legally considered theft. Access to, and through, private land must be granted by the landowner prior to use. Please be aware of all federal, state, and local laws; and be sensitive to the landowners wishes.
Joining a local gem and mineral club is an excellent way to go collecting on club-sponsored field trips and/or club-owned claims. See the RockTalk right-sidebar for links to many resources including many gem and mineral clubs around the state.