This one is annoying to get.
Shards - Drops from Skeletons through World 4.
Chunks - Drops from Skeletons through World 4. Drops from Dual Katana Black Skeletons in 4-1/4-2. Drops from Golden Skeletons in 4-2. Drops from Black Phantom Dual Katana Skeletons/ Black Phantom Gold Skeletons in 4-2 during Black World Tendency.
Pure - Extremely Rare Drop from Dual Katana Black Skeleton in Hidden passage in 4-2. Extremely Rare drop from Black Phantom Dual Katana Skeleton in Pure Black World Tendency in 4-2.
Information gotten from
http://demonssouls.wikidot.com/upgrade-sharp