Firsts and Seconds
Trees are alive. They have insect parasites, fungi, plant viruses and bacteria, they catch cold (okay . . . I'm not sure they catch cold), sometimes get sick, sometimes they die. When they are old, the wood shows all of the history of the plant: dry periods, wet periods, fires, insect blooms, and any number of other insults to their growth and development.
When mature trees are harvested, the log is sliced into planks and dried in a kiln (takes a month or so). The moisture is reduced from about 50% to around 10% and in the process the wood shrinks like drying cloth. The enzymes and plant fungi may change color; the natural chemicals in the wood "develop" in the heat of the kiln like the colors in a photograph. In the end, the dried lumber is different than wet wood, shows numerous streaks, spots, odd patterns of grain, and the interface between the center of the tree (heart wood) and the outer portion (lighter wood).
Our Hard Maple Blocks are all made the same way at the same time in the same sizes with the same sanding and care. But some of the blocks are prettier than others. We loosely regard them as being composed of two classes of color: (1) Firsts, and (2) Seconds (there is no clear cut dividing line). When we make our Premium Sets we select only Firsts with the most even grain and color; what remain are the Firsts and Seconds composed of both types. We give a discount of about 10% for Firsts and Seconds. Many people who buy them wonder what the difference was as they are very similar.
Which should you buy? Here's my best answer: in 6-12 months you usually won't be able to tell the difference. The premium sets are really beautiful, but people who take the blocks out and inspect them all for "pretty," or "smooth," are overthinking. Blocks are a roughhouse toy for a roughhouse environment - more like a box of bolts than a feather boa. Frankly, the best target for a nice set of blocks is a 7 year old boy or girl who loves tools. They get banged about, handled with dirty hands, left outside, thrown out the second story window on a parachute. They're tough; they can take it. They last forever despite all the abuse. But they will show this history on the surface and the only way to keep them pristine is to avoid playing with them. We make them out of Hard Maple so they will trooper-on through rough play and shrug off the abuse, not so people can sit around and admire them.
Then again - if you want Premium Color blocks - the difference in price isn't that great. For about 10% more you get a creamy even color.