The bottle seemed to made of copper, and the liquid shone bright like a diamond when it left the bottle.
In the 2005 film from Disney/Walden Media, the bottle appears as a silver flask, and the liquid is coloured turquoise.
While the exact nature of the bottle and it's contents were never fully revealed, the liquid had the ability to conjure food and drink, apparently, out of thin air. When Edmund Pevensie first encountered the White Witch, she enticed him with a hot drink and Turkish Delight to eat. To create each of these victuals, she poured a single drop from the bottle onto the snow on the ground, from whence the items appeared.