Its a fine grained hardwood - very popular in late 1800s for furniture, and again in the 70s & 80s, as both durable and beautiful. Then the supply of that timber (from old growth forests) bottomed out and imports from Indonesia/Malaysia pretty much took over the furniture market. Your pen may well be repurposed timber from old/antique furniture, as new timber of that variety doesn’t often come onto the market now, and what can be cut under the current strict licence conditions mostly goes for musical instruments. Treasure it - it’s beautiful.