The following small egg-like rock was found laying on the ground in Tampere, a city in southern Finland. It weights about 950 grams, has a volume of about 360 cm3 and thus a density of about 2600 kg/m3. It's longest diameter is about 140 mm. The outer grey layer is about 2 to 3 mm thick. The core looks like ironstone. The hardness of both the outer layer and the core are more than 2.5 and less than 5.5 on Mohs scale. The flattest side of the rock has a thin layer of some black soft substance, maybe asphalt. A compass or strong magnet did not react to the stone in any way and neither did a geiger counter. The rock looks like a fossil egg but I guess there is a more probable explanation. Could you be able to identify what this rock is and in which kind of a geological process such rocks can be formed?
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