Silk has a multi-millennium history as an expensive fiber used to weave luxury garments, textiles, and rugs. Archaeological evidence records silk cultivation in China as early as 6,000 BCE. Spun by several insects including spiders, the silk used in rugs is produced by commercially raised moth caterpillars. The best-known silkworm is B. mori known as the Mulberry silkworm. Silk cultivation travelled from China to Asia and the Islamic world, and by the 15th century was well-established in parts of the West. The triangular molecular arrangement of silk allows it to reflect rather than absorb light and creates an iridescent sheen that make silk rugs seem to shift in color when viewed at different angles. Silk fiber takes dye well, creating a deep saturated color. The delicacy of silk belies its relative strength while also making it slower to weave than wool but with a finer weave. Some historic silk pile rugs have cotton warps and cotton or silk wefts with the visible pile created by rows of silk knots. Sumptuous and rare antique silk rugs had gold thread diagonally wrapped around silk core fibers to highlight certain areas of a design. Less expensive but still sumptuous rugs combined silk, gold, and wool (see Wool and Silk). India was the only carpet weaving society whose preferred luxury fiber was pashmina or cashmere wool from the undercoat of the Himalayan mountain goat, not silk. Wild or uncultivated silk (tussah) was produced in Bengal and Assam, but the silk for pile carpets was imported from China or northern Iran. Some fine weave rugs had pashmina pile on a cotton foundation with the addition of a silk weft throughout. Silk and wool rugs are the best of two worlds, contrasting the reflective luminosity of silk with the robust qualities of wool (see Wool and Silk).