Silica is a chemical compound composed of silicon & oxygen atoms. Silica is a very common & naturally occurring compound due to the fact that oxygen & silicon are the most plentiful elements present in the earth's crust.
Silica is the most common mineral on earth. It is known for its hardness & is therefore often used in the production of glass products. Fiber optics for telecommunications is made from silica & it is a common additive in food production for use as a flow agent & to absorb water. One can head to DiatomaceousEarthFoodGrade.com to learn more about this supplement.
Diatomaceous Earth (DE) is a plant that exists in oceans and lakes. This deposit of DE is mined from underwater beds or ancient dried lake bottoms. This is why DE has a countless shelf life as long as it is kept dry. DE is used in pool filters and filtering and regrettably it's been heated and chemically treated and should not be used as a substitute for food grade Diatomaceous Earth.
In its food grade version it is safe for humans, pets and the environment, but is actually a potent killer of aphids, white flies, snails, slugs, fleas and ants. Further, it is rich in nutrients which are beneficial to micro-organisms and earthworms, so it doesn't protect your vegetable garden, it helps it to flourish.