The Blacksmith costs more gold to level than the Jeweler.
Much more.
At all likelyhood, you'll be easily able to keep the Jeweler at a similar level to your character (that is, making gems of the appropriate level) while the blacksmith will start to lag behind.
At any rate, the bottleneck and primary gold sink is definitely the blacksmith, so leveling up the two at the same time is definitely viable, simply because the total cost of the Jewler is essentially an afterthought.
In general, from about Normal difficulty Act 2 onwards, a +10% or higher bonus (from putting a Ruby into a socketed helmet) is going to beat the pants off of any +XP bonuses you can find.
The +XP bonus enchant tends to max out around +15 XP, which would mean it would beat a +10% XP enchant if monsters were worth less than 150 XP per kill. About midway through Act 2, I sampled some of the monsters XP values as I was killing them, by monitoring my current XP to level before and after encounters. Routinely, the average per-kill was more than 200 XP. As you progress through the game, the amount of XP per-kill will increase, and easily dwarf the small bonuses you can get from +XP gear.
This doesn't mean that you should not use +XP enchants on other equipment - but especially later in the game, it becomes less and less worth the investment. Almost always, if you have a choice between a socketed helm with a good quality ruby, or a non-socketed helm with a +XP enchant, take the helm with the ruby.
Bear in mind that the XP you gain from monsters also depends on your level versus the level of the monsters, and information about monster levels is not readily available in-game. Thus, the only real way you can say how much the monsters you are fighting are contributing to your XP is to hover over your XP bar before and after killing them, and then do the math based on these values.
Best Answer
They're ingredients for both crafting recipes and upgrading the blacksmith level. In either case, the blacksmith tab will show a Page of Blacksmithing (or other dropped ingredients) as a requirement for the action that requires a page. Just hold on to them until you get to that level.