Substance derived from silica (sand is a silica). The unique fluid properties of silicone give it a great deal of slip, and in its various forms it can feel like silk on skin, impart emollience, and be a water-binding agent that holds up well, even when skin becomes wet. In other forms, it’s used extensively for wound healing and for improving the appearance of scars. 
There are numerous forms of silicones used in cosmetic products, particularly leave-on skincare products and all manner of hair-care products. Common forms of silicone are cyclopentasiloxane and cyclohexasiloxane; other forms include various types of dimethicone and phenyl trimethicone.
Claims that silicones in any form cause or worsen acne have not been substantiated in published research, nor have reports that silicones are irritating to or “suffocate” skin. Almost all of these claims are either myths or based on anecdotal evidence, which isn’t the best way to determine the safety or efficacy of any cosmetic ingredient. How do we know that silicones don’t suffocate skin? Because of their molecular properties they are at the same time porous and resistant to air. Think of silicones in a skincare formula like the covering of a tea bag. When you steep the tea bag in water the tea and all of its antioxidant properties are released.
Silicones remain on the surface of your skin and the other ingredients it’s mixed with “steep” through. All ingredients must be suspended in some base formula; some of the ingredients remain on the surface, some are absorbed. The intent is for the “actives” to get through. Think of how many topical medications are suspended in petrolatum or mineral oil and the active ingredients absolutely get through, and petrolatum is far more effective at preventing moisture loss than silicones are.
Moreover, the molecular structure of commonly used silicones makes it impossible for them to suffocate skin (not to mention skin doesn’t breathe). The unique molecular structure of silicones (large molecules with wide spaces between each molecule) allows them to form a permeable barrier and also explains why silicones rarely feel heavy or occlusive, although they offer protection against moisture loss. 
Interestingly, silicone has been shown to be helpful for offsetting dryness and flaking from common anti-acne active ingredients such as benzoyl peroxide and topical antibiotics.  Also, silicones are sometimes used as fillers to improve the appearance of acne scars, which certainly wouldn’t be the case if silicone were a pore-clogging ingredient.  Perhaps the most telling reason why silicones do not clog pores and cause acne (or blackheads) is because, from a chemistry standpoint, most silicones are volatile. That means their initially viscous (thick) texture evaporates quickly and does not penetrate the pore lining where acne is formed. Instead, they help ensure the even application of other ingredients and leave behind a silky, almost imperceptible feel that noticeably enhances skin’s texture and appearance—without irritation.
- Klopp R, Niemer W, Fraenkel M, von der Weth A. Effect of four treatment variants on the functional and cosmetic state of mature scars. J Wound Care. 2000;9(7):319-24.
- Schlossman M. The Chemistry and Manufacture of Cosmetics. Carol Stream: Allured Publishing Corp; 2002. p. 833-839.
- Draelos Z, Callender V, Young C, Dhawan S. The effect of vehicle formulation on acne medication tolerability. Cutis.. 2008;82(4):281-4.
- Fabbrocini G, Annunziata M, D’Arco V, De Vita V, Lodi G, Mauriello M, Pastore F, Monfrecola G. Acne Scars: Pathogenesis; Classification and Treatment. Dermatol Res Pract. 2010;2010:893080.