This is one of the best hikes I have ever done. I did this hike in September of 1999 with John. The colors were changing and the peaks had been dusted with early winter snow making for wonderful scenery. The long trail starts from Christmas Meadows trailhead on the north slope of the Uintas, just a little north of the Bald Mountain/Mirror Lake area.
It is a long 6.6 miles to the lake, but the views will make it worthwhile. The trail begins by heading south nearly level for three miles. A stream will be to your right (west) all of the way. The walk is in pretty dense forest, but many spots offer open views to the west and south. A-1 Peak will be quite obvious to the west. Just before reaching the trail split at three miles you'll reach the wilderness boundary. At the split the left fork heads to Amethyst Lake, while the right for leads up Stillwater Fork and to Kermush Lake.
Almost as soon as you take the Amethyst fork, the trail steepens and climbs at a sharp grade for a mile or so, climbing nearly 1,000 feet in the process. Soon the trail levels out again and begins to bend into Ostler Fork. You'll see rugged Ostler Peak getting closer. The long ridge to the east is part of Lamotte Peak. At about five miles up the trail you'll encounter one of the most scenic meadows I've ever seen. A huge opening in the midst of the thinning forest. Should you be camping, this would surely be the place to be. Follow the trail, likely to be wet much of the year pst some tiny lakes to the east, and then up a last bench to get to the much larger Amethyst Lake.
The lake is very pretty from a variety of angles. Now at the timberline, you'll find numerous boulders to rest on. The view back down the valley you came up is pretty. Ostler Peak, now looming directly overhead to the east looks even more impressive up close. It would be a challenging scramble from the lake to this peak. The lake sits in a cirque, and crossing the steep ridge would drop you in the Rock Creek drainage. We stayed for nearly an hour enjoying all the great scenery. Moose are common in the Uintas, and we saw a pair on the hike up. Despite the climb of 6.6 miles to the lake, it only gains about 2,000 feet (most of that is in that fourth mile just after the intersection).
The hike back is a long one, and when you factor in a two hour drive to and from Salt Lake City, you can rest assured that this is an all-day adventure, but one that I'm sure you'll agree was worth your time. In fall, the Mirror Lake Highway is a beautiful drive, and this day was no exception. Be prepared to pay a small fee for parking in the Uintas in this area. At a small station just before Mirror Lake you can obtain a parking pass.