Oxalic acid and xylose crystals. Cross-polarised light micrograph of grown crystals of oxalic acid with rhamnose. Oxalic acid occurs naturally and is present in larger quantities in parsley, sorrel and rhubarb leaf. It is used in rust removal compounds, for bleaching wood and mixed with sugar syrup for treating Varroa mite in honey bees. Rhamnose is a sugar that is found in buckthorn and Uncaria species. Rhamnose extracts from the latter are used in anti-wrinkle creams. Magnification: x71 when printed 10 centimetres wide.