It's an old saw, your parents telling you to eat your gelatin because it will make your hair and nails strong. There are sites on the internet stating similar claims, but they're usually the same sort of site trying to sell the latest super-herb, so I have my doubts. As for parents, well, these are the same parents who'll tell you to eat your crusts to get curly hair. So, is there any proof that eating gelatin improves hair and nail strength? And, since it's mentioned in links above and could also be a vector, does topical treatment fare any better?
I did a quick google search on this one and the first result I got back was this link which explains the history of knox gelatin and when it started being used as a supplement to allegedly increase nail strength which was around the 1950s. Here is the main section that explains why this myth is false and why gelatin doesn't help nails.
Nails and Food
Gelatin does contain some protein. However, if protein were responsible for improving brittle, peeling nails, gelatin would be a poor choice, given the fact that other foods are much higher in protein. Eating or drinking gelatin won't strengthen weak nails, nor will soaking your hands in gelatin. Peeling nails are rarely caused by nutritional deficiencies. As long as you eat a healthy, varied diet abundant in essential nutrients, your diet won't affect nail quality. Peeling nails are more frequently caused by external factors.
Peeling nails are usually the result of repeatedly getting them wet and letting them dry again. Washing your hands and doing certain household chores — washing dishes, scrubbing the bathtub and watering the lawn — make nails brittle and dry. A better investment than gelatin is a good moisturizer that contains lanolin or alpha-hydroxy acids. Protect your nails by wearing cotton-lined rubber gloves when doing housework. Limit the number of times you change your nail polish and use acetone-free polish remover when you do.
Here is explains that Gelatin while containing some protein won't do much for nails since that isn't what is responsible for improving your nails. This is because nails are usually weakened by external factors such as nail polish, nail polish remover, and being wet for long periods of time. So no gelatin will not increase nail strength.
There is another link Here that better explains what the nails are made up of how they grow, and what determines their strength. Here is the specific portion refutuing the claim that nails can be strengthened by certain foods.
Some people erroneously believe that eating certain foods or using special creams, oils or lotions will increase the growth rate. Although the nail plate requires certain nutrients for proper growth, there is very little evidence that eating any particular food will cause them to grow faster. Creams, oils and lotions are sometimes sold as ‘growth accelerators’, although these claims are false, misleading and illegal. No cosmetic product may claim that it can alter or change any body function. These products and others are only for beautifying the nail plate, and only medical drugs can make such claims.
While this is talking about growth the same idea applies to the hardness the food you eat does not affect your nails in any noticeable way, unless you are malnourished or have a medical condition. Also while the biotin solution mentioned in the livestrong one which isn't supported but the rest of the livestrong stuff based on strength of nails is.
Edit: Updated with a new link as livestrong isn't reliable, but the new source from a educational website for nail technicians agrees.
I know about hair, not fingernails. But what Ian Gallant writes about nails is true of hair, too. Any diet with enough protein to keep you in good health doesn't need supplementation with protein. It's interesting that the maximum length for one's hair is genetic trait, as is thickness, although the latter varies widely in adult men.
The degree of curl won't change by eating bread crusts. Curly vs. straight hair determined by the shape of your hair follicles. This is a link to an explanation on Popular Science:
And a more in-depth one from Science magazine: