Aluminium oxide crystals:[10] 100 micrometres; aluminium oxide is relatively chemically inert and generally recognized as safe.[citation needed] Bristle tips: bristles are pliable, so they move with the skin allowing for aggressive treatments without added irritation.[citation needed] Diamond tips:[11] can be natural but usually synthetic for lower costs; erythema (Redness) is partially due to circulation rather than only irritation.[12] Liquid: water based sprays in combination with a rough plastic tip spray liquid against the skin to help gently exfoliate.[citation needed] Magnesium oxide[citation needed] Organic grains: used to buff and polish; made from trees, plants, agricultural crops, straw, reeds, maize, sunflower, cane sugar.[citation needed] Sodium bicarbonate and sodium chloride crystals.