Where in the spectrum in the rainbow does blue turn to indigo and then turn to violet? There is no sharp change in classification nor single metric that accurately captures this stuff despite an understandable desire to do so.
Given ROE's mention of gold, I'll start by saying if you want fiat money then there is basically negligible speculation in the coupons of bonds and their final payment as issued by sound governments. That's different from saying zero risk. You know the cashflows and can discount them. In contrast, you get a concept stock where the possibility of (or range of) revenue/E/CF/D is basically not capable of being meaningfully estimated and hence cannot estimate value beyond cash on balance sheet. That's toward the spec end.
Speculation: the forming of a theory or conjecture without firm evidence (source: Page & Brin)
...better evidence, less speculation. Having investment skill also reduces the extent of speculation in the investment decision. Hence, for a given stock, a holding may be more speculative for one investor than another who is better informed or able to make superior judgments.
So at least two things make up the issue of speculation vs the opposite definition:
1. the fundamentals
2. your ability to use them for benefit.
Somewhat unusually, 2. can also include pure TA.