West Coast Mining owns the Polka Dot Agate Mine near Madras, Oregon and they are open to visits!
If you would like to experience digging at an actual mine site you can visit the Polka Dot Agate Mine. It is approximately 35 miles NE of Madras, or 9.5 miles off Hwy 97 on NE Pony Butte Rd towards Ashwood.
They do not charge a fee for visiting, you only pay for the rocks you take at $3/per lb. for the Polka Dot. They also have other materials on site from other mining claims they have, including Amethyst Sage, and Blue Mountain Jasper.
They will even loan you a bucket or bag to collect in! But it is best to come prepared: bring buckets or bags, gloves, and small rock hammers. Remember to wear boots, a hat and to bring sunscreen and water.
The days and hours they are open can vary and it is a good idea to contact them to verify when they will be open. You can contact Dennis English, Still Rand, or Westcoast Mining at firstname.lastname@example.org, 509-522-4851 or 541-901-1445.