Silk is a fibre, just like cotton or linen or wool. You can make it into fabric of which ever weight you chose. Just think of cotton or wool: you can weave into the extremely lightweight fabrics used for shirts; or a fabric so heavy that it is suitable for blankets or upholstery. The same applies to silk, you can use it pure or combine it with other fibres: Wool, Linen. Cashmere and you can manufacture it into whatever weight you require. Equally the same weaving techniques apply: it can be tweed, twill, crepe, whatever. Your suit will not be from a silk as light as shirting material; the weight will probably be close to the cottons used for suiting. The weight might be similar, but the feel and handle will be totally different. Be aware that any silk suit will be prone to creasing and that the fall is completely different from a wool material. It hasn't got the soft drape of wool but rather hard, angular look and a hard sheen. (Think of Princess Di's wedding dress). It is somewhat difficult to describe the texture of something, without feeling the material. Go to your local tailor and ask to see some samples; they should have samples of suiting silk available.