Using the nail to pluck the string creates a sharper attack than when plucked with the pad of the finger. If the nail of the left hand is used then a more metallic sustained sound can be heard.
Violinists usually do not generally grow their fingernails as it interferes with their day-to-day playing. Using plectrums could be an option. Using a banjo fingerpick can reproduce the sound of plucking with your nail and if it is needed using a thimble on the L.H. can produce the more sustained sound. Care should be taken when using these items for sustained periods of time as violin strings were not designed to be plucked with hard objects, composers should bare in mind that a set of strings can cost £80!