The overall specimen is a concretion.
But the object at the core is tricker. It looks somewhat like a piece of fossil wood.
But on the west coast of Scotland a lot of the sedimentary rocks, eg. Torrison Sandstone, are pre-trees...
It would be useful to know where it came from (the nearest town would do).
The cross veins arose by the object cracking and the cracks becoming filled (perhaps with calcite or quartz) at the same time. The chemical changes that cause such concretions also cause changes in physical properties, and it is probably that which, as the sediments are transformed into rock, result in the cracking.
- concretions - http://www.nhm.ac.uk/natureplus/message/27257#27257
- geological map of Scotland - http://www.agatesofscotland.co.uk/The%20Geology.htm
Ah well then...
You could have a piece of the Old Red Sandstone (which was used to construct many of the older buildings in Princes Street).
The ORS covers mainly the Devonian period - ie. before the Carboniferous (where plants really got going). But there were trees in the Devonian, so the lump in the middle of your concretion could be fossil wood from one of the oldest trees.
The ORS occurs on the north (Ardrossan -to- southern Rothesay) and south (south of Ayr -to- Maybole) sides of the bay, with the middle part (Irvine -to- Troon -to- Prestwick -to- Ayr ) being composed of Carboniferous sandstones.