(as) good as gold
1. Obedient and well-behaved. Sure, I'll baby-sit Cindy's kids tonight—they're as good as gold every time I watch them.
2. Authentic and legitimate. According to the curator, this painting is as good as gold.
good as gold, as
Completely genuine; also, well behaved. For example, Her credit is as good as gold, or The children were good as gold. With this alliterative idiom the initial as is sometimes dropped, and nearly always so when behavior is referred to. [Late 1600s]