Associated minerals include quartz, feldspar, spodumene, amblygonite, tourmaline, columbite, cassiterite, topaz and beryl. Distinctive accessory minerals include&colon; fluorite, topaz, apatite, spinel, allanite, sphene and cassiterite. The state is rich in mineral deposits, especially gold, diamonds, cassiterite, bauxite, marble, and copper. There is mining of copper, iron, manganese, gold, cassiterite (tin ore), and phosphates. Goma was a center for the air shipping of cassiterite (tin oxide ore) from Nord-Kivu. Originally tin was found as alluvial deposits of cassiterite in the gravels of stream beds. Early tin exploitation appears to have been centered on placer deposits of cassiterite. Abhurite forms alongside other tin minerals like romarchite and cassiterite. This released cassiterite which was then smelted in local 'burning houses'. Bolivia's mines had produced cassiterite, the chief source of tin, since 1861. Rwanda has a nascent cassiterite mining industry. Botryoidal or reniform cassiterite is called "wood tin". The dioxide SnO (cassiterite) forms when tin is heated in the presence of air. This has led to cassiterite being considered a conflict mineral. Crystal twinning is common in cassiterite and most aggregate specimens show crystal twins.