Good quality layers pellets have a balanced diet specially formulated to contain all the nutrition including calcium and insoluable grit they need. Commercial layers are the Olympic athletes of the laying world and layers feed is formulated exactly for their needs. Commercial egg producers are exacting in their requirements and want perfect shell and yolk qualities so you can be sure the minerals they need will be in the feed. When feed is formulated all the base ingredients are analysed and any inbalances and addition minerals required are added under the control of computer programmes so that nutritionally the exact requirements are formualted.
Having said that it generally won't do any harm to make mixed grit available and older chickens may require more calcium than younger birds. I would disagree with one thing Chickenlady says it is possible to overdose chickens on calcium and the effects on the egg shells can be to cause rough sand papery chalky shells and calcium pips and the effect on the health of the chickens is not good either. Unbalancing the chickens diet by over feeding scraps corn or giving unnecessary suppliments can be just as bad as having a mineral deficiency,
If your chickens pick up gravel from the drive that's just the same as giving them grit from a feed merchants which is just crushed gravel. Oyster shell is over 90% calciium and they won't want too much of that, Crushed baked egg shell is around 35% calcium and you have to crush a lot (several hundred) of egg shells to get the equivalent amount of calcium as you'd get in a £1 worth of oyster shell, but if you've nothing better to do ?
See also this thread
http://chat.allotment-garden.org/index.php?topic=43698.msg513005#msg513005 particularly CatlinJ's post.
Regards